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1 This article was downloaded by: [Duke University Libraries] On: 21 May 2015, At: 09:26 Publisher: Routledge Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Memory Publication details, including instructions for authors and subscription information: pmem20 The Spacing Effect Depends on an Encoding Deficit, Retrieval, and Time in Working Memory: Evidence Kelly Braun David C. Rubin Published online: 15 Oct To cite this article: Kelly Braun David C. Rubin (1998) The Spacing Effect Depends on an Encoding Deficit, Retrieval, and Time in Working Memory: Evidence, Memory, 6:1, 37-66, DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of

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3 Ó M E M O R Y, , 6 (1 ), 37 ±6 5 T he Spacing E ffect D epends on an E n coding D eficit, R etrieval, and T im e in W orking M em ory: E vidence from O nce-p resented W ords K e lly B raun and D a vid C. R u bin D uke U niversity, D urham, N C, U SA T he sp acing effect in list learning occurs because identical m assed item s su ffer encoding deficits and because sp aced item s benefit from retrieval and increased tim e in w orking m em ory. R equiring the retrieval of identical item s produced a sp acing effect for recall and recognition, both for intentional and incidental learning. N ot requiring retrieval produced spacing only for intentional learning because intentional learning encourages retrieval. O nce-presented w ords provided baselines for these effects. N ext, m assed and sp aced w ord pairs w ere judged for m atch es o n th eir first three letters, forcing retriev al. T h e w o rd s w ere no t identical, so th ere w as n o encoding d eficit. R etriev al cou ld and did cause spacing o n ly fo r the first w ord of each pair; tim e in w orking m em ory, only for the second. INTRODUCTION The spacing effect is one of the oldest (Ebbinghaus, 1885/1964, p.7 9) and best studied effects in m em ory research (C row der, 1976; D em pster, 1988; G lenberg, 1979; H intzm an, 1974; R ubin, 1995). T he spacing effect can be of great practical im portance because the im provem ent of spaced over m assed practice can be large (e.g. L andauer & A inslie, 1975) and, unlike other m nem onic aids, it can be im plem ented w ithout additional study tim e or training. N onetheless, its cause rem ains a m ystery. H ere w e use three m echanism s to account for the advantages of spaced over m assed presentations over the short tim e range used R equests fo r reprints should be sent to D avid C. R ubin, at D epartm ent of E xperim ental Psych ology, D uke U niversity, D urh am, N C , U S A. E -m psych.duke.edu. T h is re se a rc h w a s su pp o rte d in pa rt b y N S F g ra nt n u m b e r B N S P o rtio n s o f this re se a rc h w e re p re se nted a t th e M e e ting of the P sy c ho no m ic s S o ci e ty. A m o re de ta ile d v e rs io n o f m ost o f this research is available in K. B raun s (1994 ) doctoral dissertation, A retrieval m odel of the spacing e ffec t, D u ke U n iv e rsity, (U n iv er sity M icro film s N o ). W e w ish to th an k J. Z e rv a k is fo r h e r help w ith the figures and G. C raik, R. C ro w der, A. G lenberg, R. G reene, & R. B lock for their c om m e n ts. K elly B raun is now at O racle C orporation Psych ology P ress L td

4 38 BRAUN AND RUBIN in m ost laboratory studies. W e m anipulate these m echanism s during a study phase and test them at the end of the experim ent using either an intentional or incidental free recall, cued recall, or recognition test. T he first m echanism is an encoding deficit during the study phase. W hen an item is repeated consecutively, subjects do not give the second presentation of the item as m uch attention or effort as they w ould to a novel item or to the second of tw o w idely separated presentations of the sam e item. Such studyphase encoding deficits affect later test-phase assessm ents of m em ory. T his m echanism gains support from studies of spacing (C uddy & Jacoby, 1982; H intzm an, 1974), overt rehearsal (C iccone & B relsford, 1976; R undus, 1971), self-paced learning (Z im m erm an, 1975), and the feeling of know ing in w hich people tend to overestim ate their ability to later recall a m assed item com pared to a spaced item (Z echm eister & Shaughnessy, 1980). In the laboratory, the subjects do not know at the tim e of the first presentation w hether and w hen the second presentation w ill occur. T herefore, the encoding deficit operates m ainly on the second of tw o presentations. In particular, the encoding deficit s effect on the first presentation is lim ited to processing that occurs after the beginning of the second presentation. T he second m echanism is retrieval during the study phase; in m any but not all situations, the second occurrence of an item causes the first to be retrieved. If an item is retrieved from long-term m em ory instead of being accessed directly from w orking m em ory, then its probability of being rem em bered later during the test phase is assum ed to increase. B ecause the phonological loop of w orking m em ory holds only tw o seconds of m aterial, w ith the procedures used here a lag of one other item being used as a retrieval cue is, as is argued in m ore detail later, usually sufficient to clear w orking m em ory and require retrieval from long-term m em ory (B addeley, 1986). Support for this m echanism com es from studies of spacing (G reene, 1989; M odigliani & H edges, 1987; T hios & D A gostino, 1976; Y oung & B ellezza, 1982) and from general m odels of m em ory (e.g. B jork & B jork, 1992). M oreover, study-phase associations can be established betw een an item and its cues that can aid the cue s effectiveness in a later recall (Jacoby, 1974). G reene provides the m ost recent argum ent for the effect of retrieval in spacing. H e notes that it should be especially effective in free recall because it encourages the encoding of aspects of the context that differ across conditions as evidenced by an im provem ent in the tem poral ordering of item s. In contrast to G reene, w e m ake no claim that retrieval has a differential effect on m em ory for context as opposed to m em ory for the item itself. T he third m echanism is the am ount of tim e item s spend in w orking m em ory during the study phase. T his m echanism can w ork on both the first and the second of tw o presentations. T he longer an item is being used in w orking m em ory, the greater the probability of its being rem em bered later in the test phase. Support for this m echanism com es from a w ide host of verbal learning

5 ONCE-PRESENTED WORDS 39 studies (A tkinson & Shiffrin, 1968; C raik & W atkins, 1973; Jacob y & B artz, 1972), and from m odels of prose processing (K intsch & van D ijk, 1978). H ere, tim e in w orking m em ory is varied by m anipulating the tim e an item spends as a retrieval cue w hile there is a search for a m atch w ith an earlier item. It m ay be possible in theory to collapse m uch of the encoding deficit factor into tim e in w orking m em ory, but w ith the techniques used here w e have no w ay of determ ining this, and so, for now, the encoding deficit is left as a separate factor. The three study-phase m echanism s used here to account for the spacing effect are not ad hoc. T hey are all needed to account for data elsew here in cognitive psychology, and they all have extensive em pirical support. T w o of the m echanism s, retrieval and tim e in w orking m em ory, are specific and w ell defined in m ost cognitive architectures. T he third, encoding deficit, is not. It could be caused by any of a host of factors. B ecause our m ain focus is on the spacing of once-presented w ords, w here encoding deficit is not a m ajor factor, w e w ill do little to clarify this situation. In order to integrate these three m echanism s in a m odel of spacing w e need to consider the role of once-presented w ords in understanding the spacing effect. Som e uses of once-presented w ords in understanding the spacing effect are standard. For instance, a com m on m echanism proposed to explain the spacing effect is encoding or contextual variability (G lenberg, 1979). A ccording to this m echanism the encoding context varies during the course of an experim ent. Item s that are spaced as opposed to m assed have a broader range of contexts and therefore m ore and different retrieval cues. W e did not include this m echanism because the encoding variability explanation should hold for once-presented as w ell as tw ice-presented item s, and it does not (G lenberg & L ehm ann, 1980; R oss & L andauer, 1978). If tw o different w ords are spaced on a list, the probability of at least one of them being recalled should be greater than it w ould be if they w ere m assed, but is not. A nother use of once-presented w ords has been to provide tw o baselines against w hich spacing can be m easured. If the probability of recall of a tw icepresented w ord equals the probability of recall of a once-presented w ord then the m nem onic costs and benefits of the second presentation of the tw ice-presented w ord m ust just balance. A second baseline can be form ed from once-presented w ords. R oss and Landauer (1978) noted that the probability of recall of a tw icepresented w ord could be com pared to the probability of recall of at least one of tw o once-presented w ords. If the tw o presentations of a tw ice-presented w ord are independent then the probability that at least one of their presentations is recalled should be equal to the probability that at least one of tw o different oncepresented w ords is recalled. If there is not independence, then there can be higher recall (R ubin & W allace, 1989; W atkins & K erkar, 1985), but the independence assum ption is the theoretically relevant baseline. If the tw o different once-presented w ords, a and b, are assum ed to be independent, the

6 40 BRAUN AND RUBIN probability of at least one being recalled is p(a) + p(b) ± p(a). p(b). B oth p(a) and p(b) are estim ated by the average probability of a once-presented w ord being recalled, p, so this reduces to 2. p ± p 2. R ecall equal to 2. p ± p 2 indicates that the m nem onic costs and benefits of the second presentation of the repeated w ord equal that of an unrepeated w ord. For these reasons, once-presented w ords are included in the first tw o experim ents. In order to use once-presented w ords as a baseline, they should be of the sam e difficulty as the tw ice-presented w ords. H ere, w e approxim ate this by assigning the sam e w ords to different subjects as either a once- or tw ice-presented w ord and by m inim ising serial position effects by placing the once- and tw icepresented w ords in pseudo -rando m orders in 12 5-w o rd lists w hic h are surrounded by 10-w ord buffers. In addition to these existing uses of once- presented w ords to calculate baselines against w hich to m easure effects, they can also be used to form m assed and spaced pairs. The spacing effect is usually considered w ith identical item s presented tw ice. B ut the item s need not be identical. T o argue otherw ise is to argue that the basic m echanism s of cognition change w hen the sam e item is repeated. R ather the sam e theory m ust account for the spacing effect in identical and different item s, although specific m echanism s m ay be needed for either identical or different item s. For instance, encoding deficits should be absent in the incidental learning of different item s. In the m ain m ethodological contribution of this paper, once-presented w ords are used to separate the effects of tw o of the three hypothesised m echanism s. If different but related w ords are included in a list, they can be m ade to function as pairs for the subjects. B ut because they are different, w e can attribute any change to either the first or the second m em ber of a pair. O nly the first m em ber of the pair can benefit from retrieval. O nly the second m em ber of the pair varies the tim e it spends in w orking m em ory as a cue for retrieval as a function of lag. It is possible to infer differences betw een the first and second presentations in the spacing effect by using the sam e w ord if subjects judge the frequency of presentation of m odalities and one presentation is auditory and one is visual (H intzm an, B lock, & Sum m ers, 1973) or if the w ords are presented in different colours (W atkins & K erkar, 1985), but the use of different w ords allow s a sim pler m ore direct test. In E xperim ent 3, during the study phase, w e have subjects judge w hether different w ords m atch on their first three letters. T his variant of a continuousrecognition task forces the retrieval of the first item if it is not in w orking m em ory; that is, if it is spaced. T herefore increases in the recall of the first (but not the second) item w ith spacing can be attributed to retrieval of that first item. B ecause the second item spends m ore tim e in w orking m em ory if it is being used as a probe or cue to search for a spaced as opposed to a m assed first item, increases in the recall of the second (but not the first) item w ith spacing can be attributed to the second item s increased tim e in w orking m em ory. T hat is, the

7 ONCE-PRESENTED WORDS 41 use of different w ords m atching on their first three letters allow s us to isolate retrieval from tim e in w orking m em ory. A s long as the occurrence of the second item is unpredictable to the subject, encoding deficits can only occur after the second item is presented. T he second item can, how ever, serve to stop processing on the first item, or a m echanism could be postulated that depends on the tem poral overlap of the processing of both item s (Landauer, 1969). If the tw o item s are different, no encoding deficit should occur under incidental learning conditions because there are no encoding processes that should be reduced. T hat is, there is no prediction of encoding deficit under m echanism s such as consolidation, habituation, rehearsal, or voluntary attention (H intzm an, 1974). U nder intentional learning conditions, how ever, subjects m ay choose to reduce controlled processing such as rehearsal, or voluntary attention. C onsider a m odel of the spacing effect that integrates the three m echanism s of encoding deficit, retrieval, and tim e in w orking m em ory. Start w ith an incidental m em ory task during the study phase to m ake som ew hat plausible the assum ption that our subjects perform the processes needed to do the cover task w e set them, and only those processes. For sim plicity, let us start w ith a continuous recognition study-phase task (like that to be used in E xperim ent 1) in w hich the subjects have to decide w hether or not the w ord currently show n on a com puter screen has occurred earlier in the task. A fter som e buffer item s, the sequence, shadow, shadow, volum e, alcohol, volum e, appears at a four-second rate. For the once-presented w ord, alcohol, there is no encoding deficit because the w ord has not occurred before. T here is no retrieval of this w ord w hen it appears because it has not appeared before. T here is considerable tim e spent in w orking m em ory as the w ord is used as a cue to search the earlier list in a vain attem pt to find a m atch. In our procedures it w ill turn out that deciding correctly that a w ord did not occur earlier w ill take about as long as deciding correctly that it did occur if it is the second w ord of a spaced pair. B oth w ill take about tw o seconds or about the tim e needed to clear the phonological loop of w orking m em ory. T he level of free recall or recognition of this once-presented w ord during a surprise test phase, w hich follow s the continuous-recognition study phase, provides our baseline level. For the m assed presentation of the item, shadow, the first presentation w ould result in the sam e effects and therefore the sam e level of initial learning as the once-presented w ord. T he second presentations w ould suffer som e degree of encoding deficit, w ould add no retrieval effect because the w ord that needs to be recognised is still in w orking m em ory, and w ould not benefit m uch from added tim e in w orking m em ory because the recognition m atch w ould occur quickly. Thus, if the encoding deficit w ere com plete for the second presentation, and the added active tim e in w orking m em ory w ere negligible, the m assed presentation w ould provide equivalent perform ance to the once-presented w ord in a later surprise recall or recognition test given at the end of the experim ent. Increases

8 42 BRAUN AND RUBIN above this level can be attributed to a less than com plete encoding deficit of the tim e spent in w orking m em ory. C onsider next the spaced presentation of the w ord, volum e, at a lag of one. T hat is, one w ord occurs betw een the tw o presentations of volum e. H ere w e w ould expect less encoding deficit than the m assed condition because another w ord has intervened. There w ould be a retrieval of the first presentation of the w ord, volum e, because w orking m em o ry has b een cleared o f the first presentation of the w ord, volum e, by the intervening w ord, alcohol, and the search through long-term m em ory for a m atch for that w ord. Finally, tim e in w orking m em ory needs to be considered. For the first presentation of volum e, there should be as m uch tim e as a once-presented w ord because a full search w ithout finding a m atch w ould be m ade. For the second presentation of volum e, there should be m ore tim e than for a m assed w ord because volum e w as used as a cue to search long-term m em ory. T hus the spaced presentation should result in greater recall at the end of the experim ent. T he clearing of w orking m em ory w ith one intervening item m ay seem odd to those used to thinking in term s of a 30-second short-term m em ory, but given estim ates of less than tw o secon ds for storage in the phonological loop of w orking m em ory (B addeley, 1986) and the search and m atch function being perform ed, this is a reasonable assum ption. A ssum e volum e is on the screen and the subject tries to m atch it w ith earlier w ords. In our experim ent, for this exam ple, the subject w ould have just spent four seconds w ith the w ord alcohol on the screen. W e w ill report later that for approxim ately the first tw o of those four seconds the subject w ould have been searching for earlier m atches for alcohol. M any w ords w ould have been retrieved to attem pt the m atch to alcohol, in m ost conditions enough to clear the phonological loop of w orking m em ory of its storage of volum e. M oreover, clearing of w orking m em ory w ith one intervening item is consistent w ith the finding in the laboratory spacing literature that there are large differences betw een lag zero and one (the spacing effect) but sm all, often non-significant, difference am ong lags longer than zero (the lag effect) (D A gostino & D er em er, 1973; H intzm an, 1974; T oppino & G racen, 1985; U nderw ood, 1970; W augh, 1970). If the sam e list w ere repeated, but the task given the subjects w as to judge w hether the w ords had six or seven letters (like that to be used in E xperim ent 2), then the results of a surprise test at the end of the experim ent w ould be different. If it is as easy to count letters as to rem em ber past presentations, there w ould be little reason to expect a w orking-m em ory or encodin g-deficit difference betw een m assed and spaced pairs, and there w ould be no need for a necessary or effortful retrieval of the first item given the second. T hat is, from the three m echanism s presented here, no spacing effect is expected and both m assed and spaced presentations should be near the 2p ± p 2 level. If the subjects knew that a recall test w as to follow, the results w ould again be different. E xtra processes w ould occur that w ould yield higher levels of recall

9 ONCE-PRESENTED WORDS 43 and in the 6/7-letter task there w ould be a spacing effect becau se there m ight be a voluntary, controlled encoding deficit, and intentional learning processes w ould certainly involve noting w hether a w ord occurred earlier on the list, thereby reinstating the retrieval and tim e-in-w orking-m em ory process for spaced presentations. Finally consider w hat w ould happen for incidental learning using a m odified continuous-recognition task (like that to be used in E xperim ent 3) in w hich the second occurrence of shadow is changed to shatter, the second occurrence of volum e is changed to voltage, and in w hich subjects are asked to judge w hether w ord pairs m atch on their first three letters. B ecause the w ords are different and because there is no intentional learning, there is no reason to expect an encoding deficit in any condition. R em oving any encoding deficit allow s us to focus on the tw o m em ory-increasing m echanism s. The difference betw een a m assed and a spaced presentation for the first m em ber of the pair of item s is that the w ord from the spaced presentation w ould benefit from a later retrieval. T he difference betw een a m assed and a spaced presentation for the second m em ber of the pair of item s is that the w ord from the spaced presentation w ould benefit from a longer tim e in w orking m em ory w hen it w as being used as a cue during the search for the retrieval of the first item. Finally, the difference betw een the first and second item s of a m assed presentation is that the second item w ould suffer from less tim e in w orking m em ory as a cue during the search for the retrieval of the first item. A s no m echanism has added or subtracted to the first item of the m assed pair, it should be com parable to the once-presented w ords. If the tw o w ords of each pair had been identical, the effects of encoding deficit w ould return, w e w ould not be able to assign the effects of retrieval to the first m em ber of the pair, and w e w ould not be able to assign the effects of tim e in w orking m em ory to the second m em ber of the pair. N onetheless, the sam e theory w ould account for identical and different w ords w ith m inor variations caused by the details of the task. In particular, because m atching identical w ords is easier than m atching w ords on their first three letters, there m ight b e slightly sm aller differences betw een m assed and spaced pairs in the effects of retrieval and of tim e in w orking m em ory. T he three experim ents that follow test this m odel by creating the situations just described. T he m odel is qualitative in that w e have no w ay to assess, a priori, the relative contributions of each of the three com ponents in general or in the specific procedures used here. It does, how ever, m ake m any specific, binary predictions because, as just outlined, across various study-phase conditions each of the three com ponents is claim ed to have either a m easurable effect or no effect on later test-phase m em ory. The choice of tasks just illustrated serves not only the purpose of testing the proposed m odel, but also of excluding other recently proposed m echanism s. In particular, C hallis (1993) has argued that deep processing is needed for the spacing effect in cued recall. T he tasks used here of determ ining the num ber of

10 44 BRAUN AND RUBIN letters in a w ord or judging w hether w ords m atch exactly or on their first three letters are as close to prototypical shallow tasks as one can get (C raik & L ockhart, 1972). T hus obtaining spacing here w ould be data inconsistent w ith the im portance of deep processing in the spacing effect. Sim ilarly G reene (1989) has argued that the spacing effect in free recall depends on study-phase retrieval w hereas the spacing effect in cued recall and recognition depend on an encoding deficit. M otivated by this distinction w e have included free recall and recognition tasks in the first tw o experim ents even though this m eans that in setting up the experim ent w e have to carefully balance possible floor effects in recall w ith possible ceiling effects in recognition. EXPERIMENT 1 E xperim ent 1 ensures that w e can observe the spacing effect w ith pairs of identical w ords w hen there is forced retrieval of the first m em ber of each pair. Method M aterials. A set of 75 target w ords and 20 buffer w ords w ere used. T he w ords w ere nouns w ith either six or seven letters and a m em orability score betw een.4 5 and.5 5 as norm ed in R ubin and Friendly (1986). The study list had a total of 145 presentations com posed of 10 prim acy buffer w ords, 10 recency buffer w ords, and 125 presentations of target w ords. T hese 125 presentations consisted of 50 target w ords presented tw ice and 25 target w ords presented once. T he study list w as constructed so that every subject saw the sam e order of spacings; how ever, the w ords at those spacings w ere varied in three versions of the list. The order of spacings w as determ ined by creating five uniqu e lists of 25 w ord slots, each containing five once-presented w ords and tw o pairs of tw icepresented w ords at each of the five lags (i.e. num ber-of-intervening-item s) conditions. In these 25-w ord-slot lists the placem ent of the five different lags (lag 0, 1, 2, 4, and 8) appeared in different serial locations. T hese five lists w ere random ly appended to each other to create a 125 w ord-slot list for target w ords. T o create the final three versions seen by the subjects, w ords w ere random ly draw n for the target w ord list w ith the stipulation that each w ord appear in one of the three lists as a single presentation and in the tw o other lists at different lags. T he buffer w ords rem ained constant and in the sam e order in all versions of the study list. A forced choice recognition test follow ed the presentation of all the w ords for half the subjects. E ach target w as paired w ith four distractor item s. A ll five choices began w ith the sam e letter. T he distractor item s for the forced choice test w ere chosen random ly from w ords in K ucera and Francis (1967) that had the sam e first letter and frequency as the target w ord. If there w ere not enough w ords w ith the sam e frequency, a range of frequencies w as used. T o determ ine the range, the frequency of the target w ord plus and m inus one w as used first. In

11 ONCE-PRESENTED WORDS 45 a few cases of very high-frequency target w ords the rule used for determ ining the range w as from one half to tw ice the target w ord s frequency. T he few target w ords that had a frequency of zero in K ucera and Francis w ere m atched w ith w ords that had a frequency of one. T here w ere tw o different random orders of this recognition test. D esign and P rocedure. Intentionality of learning ( intentional versu s incidental) and type of m em ory test (recall versus recognition) w ere betw eensubjects factors. Spacing (m assed versus spaced) and num ber of presentations (once versus tw ice) w ere w ithin-subject factors. T he 48 D uke undergraduates, w ho participated as part of a course research requirem ent, w ere assigned to conditions in a revolving order and w ere tested individually. T he experim enter w as in the room but not in the subject s view w hen the subject w as w atching the screen of an IB M -com patible personal com puter. Subjects w ere presented w ith one w ord at a tim e at the centre of the com puter screen using the M E L psychology experim ent environm ent. T he w ord appeared for four seconds during w hich tim e the subject had to read the w ord out loud and respond by pressing the correct key on the com puter s keyboard. T he response keys w ere labelled w ith stickers printed either yes or no. T he w ord rem ained on the screen for four seconds no m atter how quickly the subject responded. B etw een each w ord a fixation cross w as presented for one second. Subjects w ere inform ed that there w ere w ords repeated w ithin the list and that presentation of the list w ould take approxim ately 10 m inutes. For the continuous-recognition task, subjects w ere asked to decide if the w ord on the screen had been presented in the list before. The correct answ er w as yes for the second occurrence of each of the tw ice-presented w ords and no for the first occurrence and for all of the once-presented w ords. Subjects, w ho w ere given a practice list of 10 w ords, could ask questions about the procedure and the experim enter corrected them if necessary during the practice list. A ll subjects dem onstrated an understanding of the task before they w ere given the experim ental list. The instructions, w hich w ere read by the experim enter, differed according to the intentionality of learning. Subjects to the intentional learning conditions w ere inform ed that they w ould be asked to produce as m an y w ords as they could rem em ber after the entire list w as presented. Subjects in the incidental-learning condition w ere not inform ed of a later test. T hese subjects thought the w orddecision task w as the only task. In this experim ent as w ell as all others reported here, subjects in incidental conditions show ed surprise w hen given the retention test. A fter the study-phase list, all subjects in the free-recall test-phase condition w ere given a blank, lined sheet of paper and asked to w rite dow n as m any w ords as they could rem em ber from the list. T hey w ere told they had 10 m inutes to com plete the task. H ow ever, no subject produced any w ords after the first five

12 46 BRAUN AND RUBIN m inutes. Subjects in the recognition test-phase condition w ere given a m ultiplechoice test and asked to circle the w ord they thought w as previously presented in the list. T hey w ere told to guess if they w ere not sure. Scoring of D ata. T he data collected w ere the num ber of target w ords correctly recalled or recognised. B uffer w ords recalled w ere not scored in recall or tested in recognition. For this and all experim ents that used continuous recognition, responses w ere checked to ensure that second occurrences of w ords w ere recognised as repetitions. W ords not recognised during the study phase w ere elim inated from analysis during the test phase because it could not be assum ed that they produced a retrieval of the first presentation of the w ord. Results S tudy-pha se P erform ance. Subjects w ere accura te at the continuo us recognition tasks, m aking a total of 1.8 % errors (106 out of 6000 decisions). O f these 106 incorrect responses only 23 w ere w ords not correctly identified as repetitions of a previously presented w ord. T hese 23 responses (0.3% ) w ere rem oved from the analysis. N o analysis of errors w as done on these few errors. Spacing Effects. In this experim ent and all that follow, the term m assed is used for zero lag and spaced is used to m ean the average of all non-zero lags. H ere and in the literature a robust m assed versus spaced (zero lag versus nonzero lags) difference is often observed w ithout a consistent linear trend or significant differences am ong the non-zero lag conditions. T he results of such lag, as opposed to spacing, effects w ill be considered after all the experim ents are presented. M eans are show n in Fig. 1. A 2 (spacing: m assed versus spaced) 2 (intentionality: intentional versus incidental learning) 2 (test type: recall versus recognition) A N O V A w as perform ed on the tw ice-presented w ords. Statistics are based on percent correct. U nless noted as not significant (ns), all results throughout the paper are statistically significant at P <.05. T he m ain effects w ere: spacing F (1,4 4) = , M S e = , intentionality F (1,4 4) = 2.46, M S e = , ns, and test type F(1,4 4) = , M S e = N o interactions w ere significantð all F(1,4 4) s < T he lack of an intentionality effect m ay be due to a ceiling effect in the recognitio n data. In hindsight w e w ere too am bitious w hen w e contrasted free recall and recognition in the sam e study. This forced us to have either close to ceiling or close to floor effects on one of the m easures, even though w e m ade the recognition task as hard as w e could. A lthough these ceiling effects m ay have rem oved the intentionality effect, the reader should note that w e observed spacing effects even w here this problem w as at its w orst. M oreover, w ith respect to spacing, w e had large m ain effects w ith no interactions, w hich is not a classic ceiling effect pattern. T hus, the

13 ONCE-PRESENTED WORDS 47 FIG. 1. Percentage of w ord s free recalled or recognised on the final test in Experim ent 1 as a fu nc tion of w h et he r a w o rd w a s p re se nted on c e, tw ic e m a sse d, or tw ic e sp a c e d, o r th e pe rc e n ta ge th a t at least one of tw o unrelated w ords w ould be rem em bered w as calculated (calculated tw ice unrelated ). The subjects, w ho judged w hether the w ord being show n had occurred earlier in the list, w ere either aw are (intentional ) or unaw are (incidental) that a m em ory test w ould fo llow w hile m a king th e jud g em e n ts. effortful retrieval forced by the study-phase continuous-recognition task appears to be sufficient to induce a spacing effect independent of the intentionality of learning. Single versus Twice-presented W ords. A lso show n in Fig. 1 are the prob abilities of recall and rec ognitio n of once-presented w ords and the probabilities of at least one of tw o once-presented w ords being recalled, as calculated from 2. p ± p 2. T here is not a significant difference betw een the

14 48 BRAUN AND RUBIN probability of rem em bering a once-presented w ord and a m assed tw icepresented w ord, F(1,4 4) = 0.0 3, M S e = , there is an effect of test type, F (1,4 4 ) = , M S e = , b u t n o t a n in te ra c tio n, F (1,44 ) = 0.0 0, M S e = In addition, the value of 2. p ± p 2 is greater than the spaced tw ice-presented w ords, F (1,44) = 4.36, M S e = , there is an effect of test type, F(1,4 4) = , M S e = , but not an interaction, F (1,4 4) = 0.17, M S e = T hese results im ply that the probability of at least one of tw o different w ords being recalled is greater than the probability of a tw ice-presented w ord being recalled, and if the tw ice-presented w ord has a m assed presentation its probability of being recalled drops to approxim ately w hat it w ould be w ith only one presentation. T hus, overall spacing does not increase the probability of recall above that of recalling at least one of tw o different w ords, but does help to m inim ise the deficit. C om pared to tw o different w ords, the m assed presentation of identical w ords suffers both an encoding deficit and also less tim e in w orking m em ory because it does not take as long for the second presentation to be judged in the continuous recognition task. For the spaced presentation of identical w ords, both the detrim ental effects of the encoding deficit and tim e in w orking m em ory are less, because the intervening item s require an increase in encoding and a larger search tim e to determ ine that the second presentation w as a repetition. In addition there is a benefit to the first presentation from retrieval. EXPERIMENT 2 E xperim ent 2 is the sam e as E xperim ent 1, except that the continuousrecognition study-phase task is replaced by a 6/7-letter judgem ent task. A ll other aspects of the design are identical. T he continuous-recognition task forced the effortful retrieval of the first presentation of each tw ice-presented w ord at the tim e of the w ord s second presentation. T he 6/7-letter judgem ent task does not, but such a retrieval w ould be expected if the subjects knew that a later recall w ere expected. T hus, as predicted by the m odel, the second experim ent tests w hether the spacing effect occurs w ith intentional but not incidental learning using the 6/7-letter task and that the sam e effects hold for both recall and recognition. Method T he identical m aterials, design, and procedure w ere repeated except that the continuous-recognition judgem ent w as replaced w ith a 6/7-letter task and that 48 D uke undergraduates w ho did not take part in E xperim ent 1 w ere recruited. O ne half of the subjects w ere asked to decide if each w ord contained six letters and the other half decided if each w ord contained seven letters.

15 ONCE-PRESENTED WORDS 49 Results Study-phase P erform ance. Subjects w ere accurate at the letter judgem ent task m aking only 2.9% errors (174 errors out of 6000 decisions). T hese item s w ere not included in further analyses. Spacing E ffects. M eans for these m ain effects and interactions are show n in Fig. 2. A 2 (spacing: m assed versus spaced) 2 (intentionality: intentional versus incidental learning) 2 (test type: recall versus recognition) A N O V A w as perform ed on the tw ice-presented item s. T he m ain effects w ere: the notsign ific an t e ffec t o f sp ac in g, F (1,4 4 ) = 1.8 5, M S e = ; inten tio na lity F(1,4 4) = 9.0 0, M S e = ; and test type F (1,4 4) = , M S e = The 2-w ay interaction of spacing intentionality F (1,4 4) = 5.0 4, M S e = , w as the only significant interaction±all other F(1,4 4)s < A n exam ination of Fig. 2 indicates that the lack of a m ain effect for spacing and the spacing by intentionality interaction appear to be caused by the predicted outcom e of no spacing in the incidental condition w hich did not require or encourage the retrieval of the first presentation of the tw ice-presented item s. In fact, the difference betw een the m assed and spaced incidental conditions is in the w rong direction for a spacing effect. T o test this further, the data from the intentional and incidental conditions w ere analysed separately. For the intentional data there a re significant spacing an d test-typ e effec ts, F ( 1,2 2 ) = 5.2 8, M S e = and F(1,2 2) = , M S e = , respectively, but no interaction, F(1,2 2) = 1.5 4, M S e = For the incidental data there is a significant test-type effect, F(1,2 2) = , M S e = , but not a significant spacing effect F(1,2 2) = 0.5 1, M S e = or interaction, F (1,22) = 0.2 7, M S e = T he lack of a spacing effect in the incidental-learning recall condition could be considered as due to a floor effect, but this is unlikely to be the reason. First, sim ilar levels of recall produced spacing effects in E xperim ent 1. Second, recognition also failed to show a spacing effect in incidental learning and its levels are far from floor or ceiling effects. N onetheless, in the rem aining experim ent w e sw itch from free to cued recall to bring the levels of recall and recognition closer together in order to avoid possible floor and ceiling effects. Sim ilarly, it cannot be a lack of pow er that led to the lack of a spacing effect in the incidental learning recall and recognition conditions. First, the m eans are not in the correct direction. Second, there is a spacing effect for the intentional condition. Third, the overall spacing effect in E xperim ent 1, w hich differed only in the study-phase cover task and thus w hich had the identical design and num ber of subjects, w as F(1,4 4) = , P < T hus, there is certainly pow er enough in the design to detect spacing at the.0 5 level. Single versus Twice-presented W ords. Figure 2 also show s the probability of recall and recognition of once-presented w ords and the probability of at least

16 50 BRAUN AND RUBIN FIG. 2. P ercen tage of w ords free recalled or reco gnised on the final test in Experim ent 2 as a fu n c tio n o f w h et he r a w ord w a s p re sen te d on c e, tw ic e m a sse d, or tw ic e sp a c ed, o r th e p e rc e nta ge th at at least one of tw o unrelated w ord s w ould be rem em bered w as calcu lated (calculated tw ice unrelated ). T he subjects, w ho judged w hether w ords had six or seven letters, w ere either aw are (in te ntio na l) o r u n aw a re (in c id e nta l) th a t a m e m o ry te st w o uld fo llo w w h ile m a k in g the jud ge m e nts. one of tw o once-presented w ords being recalled, as calculated from 2. p ± p 2. For the incidental data, in w hich there is no spacing effect, m assed pairs are recalled m ore than once-presented w ords, F(1,2 2) = , M S e = , there is an effect o f te s t ty p e, F (1,2 2 ) = , M S e = , b u t n o t a n in te ra c tio n, F(1,2 2) = 1.4 1, M S e = In addition, the value of 2. p ± p 2 is greater than the special tw ice-presented w ords [spaced versus 2p ± p 2, F (1,2 2) = 15.17, M S e = 33.14, and there is an effect of test type, F(1,2 2 = , M S e = , and their interaction, F(1,2 2) = , M S e = ]. T o interpret their interaction, separate analyses w ere done for recall and recognition. For recall there is

17 ONCE-PRESENTED WORDS 51 no difference betw een the value of 2. p ± p 2 and the spaced tw ice-presented w o rd s, F (1,1 1 ) = 0.0 6, M S e = , w he re as for reco gn ition there is a difference, F(1,1 1) = 21.52, M S e = If the 6/7-letter judgem ent task in the incidental learning condition is done by judging each w ord separately then the recall level of a m assed presentation, a spaced presentation, and at least one of tw o different w ords should all be the sam e. M oreover, the recall level of a m assed presentation should be greater than that of a single presentation. T his occurs for recall and for recognition to the extent that, unlike Experim ent 1, the m assed presentation is greater than that of a single presentation. T he difference betw een the spaced presentation and the 2. p ± p 2 calculation for recognition is the only discrepancy from the predicted results. For the intentional data, a different pattern em erges. T here is not a significant difference betw een the probability of recall of a once-presented w ord and a m assed tw ice-presented w ord, F(1,2 2) = 0.2 9, M S e = , there is an effect of test type, F(1,2 2) = , M S e = , but not an interaction, F (1,2 2) = 3.4 7, M S e = In addition, the value of 2. p ± p 2 is not greater than the spaced tw ice-presented w ords [spaced versus 2. p ± p 2, F (1,22) = 1.1 3, M S e = , test type, F(1,2 2) = , M S e = , and their interaction, F (1,2 2) = 0.9 8, M S e = ]. B ecause intentional learning involves all the aspects of the incidental learning condition plus an attem pt to learn the w ords, there should be the sam e effects as w ere present in the incidental condition m ixed w ith the effects of trying to learn the w ords. The latter effects can be approxim ated by the continuous recognition task of Experim ent 1, because part of the effort to learn w ould involve attem pts to note w hether a w ord had occurred earlier in the list. The intentional learning results are m uch closer to those of the continuous-recognition results of Experim ent 1, the only difference being that there the 2. p ± p 2 value is not greater than that of the spaced presentation. T he overall pattern of results should occur under the m odel specified in the introduction: (a) the deficit in the encoding of m assed tw ice-presented w ords should not occur in the incidental learning conditions because the w ords w ere being processed only to count their letters, but should occur in the intentional learning conditions w here added m nem onic processing took place; (b) there should be no effortful retrieval in the inciden tal conditions for the first presentation of the spaced tw ice-presented w ords because there w as no need to retrieve it under the cover task, but there should be effortful retrieval in the intentional conditions because the m nem onic processing required it. T he one exception to this conclusion is the 2. p ± p 2 calculated value for the incidentallearning recognition subjects, w hich is too high. T hus as in E xperim ent 1, this overall pattern of results is w hat w ould be predicted from the proposed m odel of the spacing effect.

18 52 BRAUN AND RUBIN EXPERIMENT 3 H aving dem onstrated the spacing effect w hen effortful retrieval of the first m em ber of a pair occurred and the lack of a spacing effect w hen that retrieval did not occur, w e search for the precise locus of the effect. E xperim ent 3 uses pairs in w hich the first and second m em bers differ. In this w ay the locus of the spacing effect can be attributed to one or the other m em ber. T his novel m anipulation is useful in the procedures used here because study-phase retrieval can occur only for the first m em ber of a pair and only w hen it is retrieved, because it is no longer in w orking m em ory w hen it needs to be com pared to the second m em ber of the pair. In contrast, differences in tim e in w orking m em ory w ith spacing can occur only for the second m em ber of a pair because the search tim e for using the second item as a cue increases w ith lag but the search tim e for using the first item as a cue does not. E ncoding deficit is not a factor for different item s. In intentional learning there m ight be encoding differences if the subjects thought they had learned the m assed pairs better, but in the incidentallearning continuous-recognition task used here such differences are not likely. T he strategies our subjects used to increase their intentional learning above that of incidental learning are difficult to uncover. W e therefore restrict our study-phase procedures to incidental learning w ith a dem anding cover task that provides m ore specific indications of processing. M oreover, in order to force and be able to m onitor retrieval of the first presentation of a pair during the study phase, w e use only variants of the continuous-recognition cover task from Experim ent 1. If retrieval contributes to the spacing effect then it should be possible to produce the spacing effect using item s that cue retrieval even if the cueing item is not an exact repetition. E xperim ent 3 w as designed to test this hypothesis. The task for the subject is to decide if they have seen a w ord previously presented in the list that m atches on at least the first three letters. In order for the subject to correctly respond ``yes, they should retrieve the previous m atching occurrence. In contrast to the predictions m ade here, previous studies have suggested that only exact repetitions produce the spacing effect. For instance, D ellarosa and B ourne (1985) found the spacing effect w ith exactly repeated sentences but not ``gist sentences, and G lanzer and D uarte (1971) found an attenuated spacing effect w hen language w as changed in bilingual subjects. T he difference here is that w e force the retrieval of the exact earlier item even though the exact earlier item itself w as not repeated. In our first w ork using this procedure som e subjects used a strategy in w hich they only looked at the first three letters of the w ord instead of reading the entire w ord. T o elim inate the use of this strategy, w e added a few unscored cases of identically repeated pairs of w ords as w ell as three- letter com binations that occu rred three tim es. T he subjects w ere asked to choose am ong three alternatives: w hether the w ord being show n (1) m atched no earlier w ords, (2)

19 ONCE-PRESENTED WORDS 53 m atched an earlier w ord verbatim, or (3) m atched an earlier w ord only on the first three letters. T he prim acy buffer contained exam ples of all three alternatives so that subjects w ere alerted from the beginning of the experim ent that using a strategy of not reading the w hole w ord w ould not be useful. In an attem pt to raise recall percentages, a cued-recall task w as used instead of the free-recall task of E xperim ents 1 and 2. T he cued-recall test prom pted w ith the first three letters of each target w ord: the letters for w hich a m atch w ould occur. T here w as space for tw o w ords to be w ritten dow n and subjects w ere inform ed in the instructions to w rite tw o w ords if they could rem em ber seeing tw o different w ords. B ecause there should be no encoding-deficit effect w ith the different-w ord pairs, w e expect the spacing effect to be sm aller. T o increase the pow er of the experim ent only different-w ord pairs are used, thereby increasing their num ber. Methods M aterials. The study list w as constructed of 75 pairs of target w ords and 10 prim acy and 10 recency buffer w ords. E ach target pair begins w ith the sam e three letters and this three-letter com bination is unique to each pair. The w ords w ere draw n from the sam e pool as used in the previous experim ent. H ow ever, because subjects w ere looking for w ords that m atched on the first three letters, approxim ately one third of the w ords in the original list had to be replaced. This w as to ensure that only one w ord could be retrieved as a yes response. R eplacem ent w ords w ere rated betw een.40 and.5 8 on m em orability in R ubin and Friendly (1986). For each of these nouns, a second six- or seven-letter noun m atching on only the first three letters w as found in K ucera and Francis (1967). O bvious associations w ere avoided. Seven additional w ords w ere needed as once-presented filler item s to produce the needed spacings of the list. A ll buffer and filler w ords began w ith a unique three-letter com bination different from any of the target w ords. The w ord list w as constructed so that there w ere 15 pairs of w ords at each of the five spacings. T h is m ade for a total of 75 w ord pairs. T o ensure that the different spacings w ere presented evenly over the list, five interm ediate w ordslot lists w ere created. T hese interm ediate lists w ere created so that the least num ber of filler item s w ere needed to properly space the item s. T hese five lists w ere then appended to each other. In som e cases it w as possible to rem ove filler item s by starting the next interm ediate list w ithin the last interm ediate list w ithout disrupting the spacing. T he final w ord-slot list consisted of the 75 target pairs and seven filler item s. T hree lists w ere created by taking the final w ord-slot list and adding the prim acy and recency buffers. The buffer and filler w ords w ere the sam e for all versions of the list. N ext each pair of w ords w as random ly assigned to a different spacing and a different area w ithin each of the three lists. Finally for each of the