Outline. n Introduction. u Task analysis u Network Representations. n From Task Analysis to Network Representations

Transcription

1 Start Search me Move crsor toward me Doble click o chose optio Otlie Itrodctio Bildig Models from Task Aalysis Task aalysis Network Represetatios From Task Aalysis to Network Represetatios Costrctig a Network Represetatio Psyc wk09 1 Resselaer 2 Resselaer Isses Task Aalysis Task aalysis are freqetly carried ot, bt mathematical ad compter models of task are less freqet Why? -- Perceived difficlty i costrctig sch models Bt, task aalysis cotais most of the iformatio eeded to costrct a workig model Task aalysis is a detailed descriptio of all aspects of the task, based o observatio, iterviews, ad iferece Article focses o how to se this descriptio to costrct a model that will idetify those task compoets most critical for determiig the completio time of the task 3 Resselaer 4 Resselaer Hierarchical Task Aalysis Network Rep-- Activity Networks (fig2) Example of selectig a optio from a pop p me Achieve this goal by carryig ot operatios accordig to plas Fig 1 HTA operatios = activities Depedecies bilt-i Cocrrecy implied Task dratio = logest path throgh etwork (I.e., the critical path) Aka critical path etwork or PERT chart 5 Resselaer 6 Resselaer 1

2 Start Search me Move crsor toward me Doble click o chose optio Network Reps -- Order-of-Processig Diagrams OP Diagrams For activity etwork if crsor was already over the right me item (say by chace) the etwork wold have jst three odes 7 Resselaer Each ode represets a state; I.e., the set of processes crretly active 3 states ser searchig me (s) ser doble clickig a optio (d) doe s s d d Lie draw betwee two states ( a directed arc ) if the crret set of processes i the sccessor state is cosistet with the completio of exactly oe of the processes i the crret set of predecessor states The arc is labeled with the process that completed (s or d i the above example) 8 Resselaer OP Diagrams From Task Aalysis to Network Rep Now with 3 processes searchig me -- s movig crsor towards me -- m doble clickig a optio -- d s & m begi i parallel -- oe completes first Assme times for s & m are radom variables -- so that o some trials s completes first ad o other trials m completes first s, m m s s m s m Task aalysis emphasizes aalysis Detailed descriptio of the task compoets ad their relatioships Network modelig emphasizes sythesis Calclatios of how the compoets fit together to determie qatities sch as task completio time There are three steps i movig from a task aalysis to a model. First, the task aalysis is sed to costrct a etwork represetatio for the task. Secod, estimates of the dratios of the activities are fod i the literatre, or, if they are available, obtaied throgh Mltidimesioal Scalig. Third, the etwork model is implemeted with eqatios, or a compter program is writte for simlatios. 9 Resselaer 10 Resselaer Costrctig a Network Represetatio Plas i Task Aalysis Two etwork reps discssed i this paper Activity Networks & Order-of-Processig Diagrams Critical path etwork is special case of a activity etwork HTA is cosidered here as the most geeral represetatio of a task aalysis Used to discss the 6 types of plas Provides sprigboard for discssio of how to costrct a etwork rep Six basic types of plas fixed seqece cocrret operatios optioal completio cycles choices cotiget seqeces How are these hadled by Activity Networks ad/or Order-of-Processig Diagrams? 11 Resselaer 12 Resselaer 2

3 Plas i Activity Networks Plas i Activity Networks (cot.) 1. Fixed seqece -- (obvios) a ode is draw to represet each operatio i the HTA as a activity, ad a arrow is draw from each activity to its immediate sccessor 2. Cocrret operatios -- (obvios) 3. Optioal completio -- case i which order for exectig operatios is optioal -- oly costrait is that they all be completed May simplify traiig by teachig oe order of completio (treat a optioal completio pla as if it were a fixed seqece) Completio time sally idepedet of order, bt if ot ca represet each order of completio as a separate ode i the activity etwork (with its ow completio time) 4. Cycles -- plas that reqire a operatio or set of operatios to be repeated til a certai coditio is met Easy to estimate time per cycle, harder to estimate mber of cycles 13 Resselaer 14 Resselaer Plas i Activity Networks (cot.) Plas i Activity Networks (cot.) 5. Choice -- Whe there is a choice of operatios, oe operatio i a set of operatios is selected. For example, oe may choose to save a file o a hard disk or save it o a floppy disk 5. Choice A critical path etwork caot represet a choice, becase all the optios are ot sed, oly oe of them Awkward for ANs For task completio times per optio -- eed to draw separate AN for each optio For mea task completio times -- se OP diagram The probability of each optio is idicated o the arrow poitig to it 15 Resselaer 16 Resselaer Plas i Activity Networks (cot.) AN --> OP Diagrams 6. Cotiget Seqece -- a pla i which a operatio is ced by somethig (e.g., a alarm) other tha the fiishig of the precedig operatios Awkward for AN For task completio times per optio -- eed to draw separate AN for each optio For mea task completio times -- se OP diagram 17 Resselaer Critical path etworks (a type of AN) ca easily be sed to represet fixed seqece, cocrret ops With some loss of detail CPN ca be sed for optioal completio, ad cycles Give a CPN of these 4 pla types ad OPD ca be atomatically costrcted Caot costrct CPN for choices or cotiget seqeces -- CAN costrct OPDs of these 18 Resselaer 3

4 Plas i OPD Plas i OPD Choice A choice i HTA is represeted as a state i OPD Optios from which the choice is made are listed as activities crretly derway Differet optios take differet amots of time to evalate -- these times affect probability of chosig that optio The optio selected is cosidered to be the activity completed to exit the state -- all other optios are dropped at that poit Cotiget seqece I HTA some activity is started by somethig other tha completio of the precedig activity (e.g., a alarm sods &... ) Cosider a alarm that ca go off at ay time -- this activity Alarm Goes Off is added to each state of the OPD For each state -- oe of the ways of exitig the state is for the alarm to go off -- for the potetial alarm activity to be completed 19 Resselaer 20 Resselaer Plas -- Smmary Discssio of CPM-GOMS Fixed seqece -- exact coterparts i AN Cocrret operatios -- exact coterparts i AN Optioal completio -- ca be represeted i AN, bt ot a exact coterpart Cycles -- ca be represeted i AN, bt ot a exact coterpart Choice -- either AN or OPD GOMS Aalyzes orgaizatio of the task (basic task aalysis) & Specifies times for the basic operatios CPM-GOMS A method for modelig cocrret activities Cotiget seqece -- either AN or OPD 21 Resselaer 22 Resselaer Task Aalysis to Model -- Steps 2 & 3 Activity Network: Step 2 -- dratio of activities Secod, estimates of the dratios of the activities are fod i the literatre, or, if they are available, obtaied throgh Mltidimesioal Scalig Meas + SDs Meas bt o SDs No meas, o SDs Third, the etwork model is implemeted with eqatios, or a compter program is writte for simlatios We first discss these steps for AN (pp18-36) the for OPD (pp36-44) 23 Resselaer Step 1 -- Activity Network costrcted Step 2 -- Obtai meas ad stadard deviatios of activity dratios Will se as a test case the example of a telephoe call from GJA93 24 Resselaer 4

5 START SRT1 LTB RS a GC L1 EC a L2 EC b ECCN EC c RS b EC d END Bar Chart & Activity Network GJA93 -- Dratios? Bar Chart Activity dratios from videotapes (bechmark) ad literatre (ormative) Activity Network If two activities i the activity etwork are ot joied by a directed path, either is reqired to be fiished before the other ca start. They may be carried ot literally simltaeosly, althogh simltaeity is ot ecessary. The activity etwork does ot idicate simltaeity, becase activity dratios are radom, ad whe the task is carried ot repeatedly, two activities ot joied by a directed path might sometimes be simltaeos ad sometimes ot be simltaeos p ways throgh this etwork 25 Resselaer 26 Resselaer GJA93 -- Variability? To se Telephoe call example to prodce a model we will eed the probability distribtios of the activity dratios For example, each mose click is ot always, exactly, precisely 200 msec (from KLM); each keystroke is ot always, exactly, precisely 280 msec, & each activity from GJA93 is ot always, exactly, precisely its bechmark dratio or its ormative dratio Estimatig Probability Distribtios (i.e., stadard deviatios) Use gamma distribtio as the distribtio fctio for reactio times ad elemetary activities Skewed to the right, as are hma respose times 27 Resselaer 28 Resselaer Gamma Distribtio Coefficiet of Variatio Extreme forms of gamma type the form of the expoetial distribtio or ormal distribtio CV = stdev/mea For hma respose time, stadard deviatios ted to rage from 1/10 of the mea (rarely smaller) to abot eqal the mea (rarely larger) I.e., CVs rage from 0.1 to 1.0 Radomly assig a CV from set ( ) to each activity from GJA aalysis 29 Resselaer 30 Resselaer 5

6 START SRT1 LTB RS a GC L1 EC a L2 EC b ECCN EC c RS b EC d END Table 1 The se the CV to derive beta ad alpha -- two parameters eeded for the Gamma Distribtio 2 Activity Name & Mea CV V = (CVxMea) beta = alpha = Abbreviatio V/Mea Mea/beta Liste-to-Beep (LTB) Eter-Commad (EC) Read-Scree (RS) Thak-Cstomer () System-RT1 (SRT1) Liste-to-Cstomer (L1) Greet-Cstomer (GC) System-RT2 () Eter-Credit-Card No. (ECCN) Liste-to-Cstomer (L2) Activity Network: Step 3 -- Implemetig a Network Model By eqatios By simlatio For fixed seqece ad cocrret operatios, i the search me example, Search Me Simlatio.xls For a choice, i the save file example, Save File Simlatio.xls For a cycle ad cotigecy, i the save file with disk fll example, Save File Disk Fll Simlatio.xls Project Erestie 31 Resselaer 32 Resselaer PE -- 8 paths throgh etwork Paths 8 paths, bt how may of these are critical paths? SRT1 --> L2 --> SRT1 --> L2 --> SRT1 --> ECa --> Mea dratio of a path is sm of the mea dratio of the activities o it Mea dratio of path with the logest mea dratio is (i my simlatio over 10,000 trials) SRT1 --> ECa --> LTB --> L2 --> LTB --> L2 --> LTB --> ECa --> LTB --> ECa --> This dratio is less tha the mea time to complete the task: Also ote that the mea dratio of this path whe it is the critical path is How ca it be that?? The dratio of the logest path is less tha the mea dratio of a trial Less tha itself whe it is the critical path? 33 Resselaer 34 Resselaer PE -- 8 critical paths PE -- Paths Freqecy o CP mea dratio of CP trials mea dratio L, L, ECa, ECa, LTB, L, LTB, L, LTB, ECa, LTB, ECa, Mea dratio of logest path throgh etwork is less tha the mea dratio of the task? Logest path throgh etwork is ot always the same path That is, the path with the logest mea dratio is NOT the critical path o every trial over all trials Mea dratio over all trials is Whe a path is the critical path for a trial, it is loger tha the other seve paths Mea dratio of a path derestimates the cotribtio of that path to the average completio time of the task 35 Resselaer 36 Resselaer 6

7 PE -- Paths Criticality of a Activity Implicatios of the derestimatio Ay particlar activity, X, is o the logest path throgh the etwork o some trials, bt ot o others Hece, the criticality of a activity is the key qatity The probability that the activity is o the critical path Activities of high criticality are importat bt a activity with high criticality may be of short dratio, moderatig its overall importace Aother measre of the importace of a activity is its criticality times its mea dratio SRT1 LTB RS a GC L1 L2 EC a EC b ECCN EC c RS b EC d Total Critical Trials Mea (criticality) Criticality* Dratio 37 Resselaer 38 Resselaer Repeated Criticality (across simlatios) Criticality verss Critical Path Name Criticality Criticality (SFP) (Gray) Liste-to-beep Thak-Cstomer Eter-Commada Activity with the highest criticality i both simlatios (SFP & Gray s) is Liste-to-Cstomer2 Bt, the most critical activity is oly o the critical path of the time Eter-Commadb Eter-Credit-Card-No Liste-to-Cstomer Eter-Commadd Read-Screeb System-RT set1srt1 --> RSa --> GC --> L1 --> L2 --> ECc --> --> RSb --> Ecd --->Ed set2srt1 --> RSa --> GC --> L1 --> L2 --> ECc --> --->Ed set3srt1 --> RSa --> GC --> L1 --> ECa --> ECb --> ECCN --> ECc --> --> RSb --> Ecd --->Ed set4srt1 --> RSa --> GC --> L1 --> ECa --> ECb --> ECCN --> ECc --> --->Ed set5ltb --> GC --> L1 --> L2 --> ECc --> --> RSb --> Ecd --->Ed set6ltb --> GC --> L1 --> L2 --> ECc --> --->Ed set7ltb --> GC --> L1 --> ECa --> ECb --> ECCN --> ECc --> --> RSb --> Ecd --->Ed set8ltb --> GC --> L1 --> ECa --> ECb --> ECCN --> ECc --> --->Ed System-RT Resselaer 40 Resselaer OP Diagram: Steps 2 & 3 Step 1 -- OP Diagram costrcted by oe meas or aother 41 Resselaer 7