APPENDIX A SAMPLE DESIGN

Transcription

1 APPENDIX A SAMPLE DESIGN

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3 APPENDIX A SAMPLE DESIGN Thanh U A.1 Introduction The Kazakstan Demographic and Health Survey (KDHS) employed a nationally representative probability sample of women age The country was divided into five survey regions. Almaty City constituted a survey region by itself, while the remaining four survey regions consisted of groups of contiguous oblasts (except the East Kazakstanskaya) oblast which is not contiguous. The five survey regions were defined as follows: 1) Almaty City 2) South Region: Taldy-Kourganskaya, Almatinskaya (except Almaty City), Zhambylskaya, South Kazakstanskaya, and Kzyl-Ordinskaya 3) West Region: Aktiubinskaya, Mangistauskaya, Atyrauskaya, and West Kazakstanskaya. 4) Central Region: Semipalatinskaya, Zhezkaganskaya, and Tourgaiskaya. 5) North and East Region: East Kazakstanskaya, Pavlodarskaya, Karagandinskaya, Akmolinskaya, Kokchetauskaya, North Kazakstanskaya, and Koustanaiskaya. The oblast composition of regions outside of Almaty City was determined on the basis of geographic proximity and demographic characteristics. The South and West Regions are comprised of oblasts which traditionally have a high proportion of Kazak population and high fertility levels. The Central Region includes three oblasts in which the fertility level is similar to the national average. The North and East Region contains seven oblasts situated in northern Kazakstan in which a relatively high proportion of the population is of ethnic Russian origin and the fertility level is lower than the national average. A.2 Characteristics of the KDHS Sample In Almaty City, the sample for the KDHS was selected in two stages. In the first stage, 40 census counting blocks were selected with equal probability from the 1989 list of counting blocks created for the 1989 population census) A complete listing of the households residing in the selected counting blocks was carried out. The lists of households obtained served as the frame for second-stage sampling which is the selection of the households to be visited by the KDHS interviewing teams. In each selected household, women age were identified and interviewed. L Census materials that were in good condition could only be found for Almaty City. For the rest of the country, census materials concerning the counting blocks were not centrally available, nor were they available in all oblasts. Consequently, different sampling frames had to be constructed, separately for the other urban areas and for the rural areas. 151

4 In the rural areas, the primary sampling units (PSUs) corresponded to the raions which were selected with probabilities proportional to size, the size being the 1993 census population, published by Goskomstat (1993). At the second stage, one village was selected in each selected raion from the 1989 Registry of Villages. This resulted in 50 rural clusters being selected. At the third stage, households were selecled in each cluster following the household listing operation as in Almaty City. In the urban areas other than Almaty City, the PSUs were the cities and towns themselves. In the second stage, one health block 2 was selected from each town except in self-representing cities (large cities that were selected with certainty) where more than one health block was selected. The selected health blocks were segmented prior to the household listing operation which provided the household lists for the third stage selection of households. In total, 86 health blocks were selected. A.3 Sample Allocation Tables A. 1 and A.2 show the distribution of the population in Kazakstan to the different survey regions, according to the 1993 Demographic Yearbook ofkazakstan (Goskomstat, 1993) as folh)ws: Table A.1 Population Distribution (1993) Total Almaty City South West Central North and East Table A.2 Percent Distribution of Population (1993) Total Almaty City South West Central North and East In Kazakstan, each city or town is divided into health blocks, each of which is the responsibility of one physician. People living in the health block would go to a designated health center for service. This is where the physician in charge is located and maintains a map of the health block and even lists of households residing in the health block. The average population size of the health block is about 2,000. There are three different types of health blocks: the internist's block, the pediatrician's block, and the obstetrician/gynecologist's block, each serving a different group of patients as the names indicate. The internist blocks are largest in number (and correspondingly serve smaller groups of patients), and therefore were selected as the area sampling units for the KDHS. The literal Russian translation of internist' s block is actually therapeutical block. For the KDHS, it is referred to simply as the health block. 152

5 The regions, stratified by urban and rural areas, were the sampling strata. Therefore, there were nine strata with Almaty City constituting an entire stratum. As shown in Table A.3, a proportional allocation of the target number of 4,000 women to the nine strata would yield the following sample distribution: Table A.3 Proportional Sample Allocation Total Almaty City South West Central North and East This proportional allocation would result in a completely self-weighting sample but would not allow for reliable estimates for three regions: Almaty City, West, and Central. Results of other demographic and health surveys show that a minimum sample of 1,000 women is required in order to obtain estimates of fertility and childhood mortality rates at an acceptable level of sampling errors. Given that the total sample size for the KDHS could not be increased so as to achieve the required level of sampling errors, it was decided that the sample would be divided equally to the five regions, and within each region, it would be distributed proportionally to the urban and the rural areas. With this type of allocation, demographic rates (fertility and mortality) could not be produced for the regions. Table A.4 shows the proposed sample allocation. Table A.4 Proposed Sample Allocation Total Almaty City South West Central North and East The number of sample points (or clusters) to be selected for each stratum was calculated by dividing the number of women in the stratum by the average "take" in the cluster. Analytical studies of surveys of the same nature suggest that the optimum number of women to be interviewed is around in each urban cluster and in each rural cluster. If on average 20 women in each urban cluster and 30 women in each rural cluster were to be interviewed, then the distribution of sample points would be as follows: The number of clusters in the South Region in Table A.5 would yield a slightly smaller number of women than expected because of rounding errors. Consequently, the number of clusters were rearranged in each stratum so that it was an even number, but in such a way that the expected regional sample size did not fall short of the required 800 minimum. The even number of clusters is recommended for the purpose of calculating sampling errors in which the first step is to form pairs of homogeneous clusters. 153

6 Table A.5 Number of Sample Points Total Almaty City South West Central North and East Table A.6 Proposed Number of Sample Points Total Almaty City South West Central North and East The number of households to be selected for each stratum was calculated as follows: Number of HHs = Number of women Number of women per HH x Overall response rate According to the 1989 census, the proportion of women age in Kazakstan was 25 percent. By applying this figure to the average household size of 4.0 obtained from a household survey conducted by Goskomstat, the number of women age was estimated to be 1.0 per household. The overall response rate was assumed to be 90 percent (95 percent for households and 95 percent for women), which was the average overall response rate found in DHS surveys. Using these two parameters in the previous equation, approximately 4,500 households had to be selected in order to yield the target sample of women. This resulted in selecting on average 22 households in each urban cluster and 33 households in each rural cluster. A.4 Stratification and Systematic Selection of Clusters Stratification of the area sampling units was mostly geographic within each sampling stratum. A.4.1 Almaty City After ordering the raions geographically, and maintaining the order of the counting blocks within the raion, the counting blocks were selected with equal probability. Selection with probability proportional to size was not necessary since the counting blocks were relatively uniform in size (average population size of 417, standard deviation of 36, and coefficient of variation of 8.6 percent). 154

7 The selection interval was calculated as follows: 1 = where 2,515 is the total number of counting blocks in Almaty City and 40 is the number of counting blocks to be selected. The counting blocks to be selected were the ones with the following serial numbers: R, R+I, R+2I,... R+391, where R is a random number between 1 and 1. A.4.2 Other urban areas In the other urban areas, the cities and towns were selected with probabilities proportional to size, the size being the 1993 population count. Large cities, or self-representing cities, that had to be selected with certainty (probability = 1.0) were separated out before towns were selected. The limit above which a city became self-representing was calculated as follows: L = Population in stratum Number of Health Blocks to be Selected Within each city, the required number of health blocks were selected with equal probability. The selection intervals for the towns were calculated as follows: EM~ a where EM~ is the size of the stratum (total population in the stratum according to the sampling frame) and a is the number of towns to be selected in the stratum. The selection procedure consisted of: (1) calculating the cumulated size of each town; (2) calculating the series of sampling numbers R, R+I, R+21,..., R+(a-1)1, where R is a random number between 1 and 1; and (3) comparing each sampling number with the cumulated sizes. The town to be selected was the first town whose cumulated size was greater or equal to the sampling number. Within each town, one health block was selected using a random number between 1 and the number of health blocks that exist in the town. A.4.3 Rural areas In the rural areas, the raions were selected with probabilities proportional to size. One village was then selected within each raion using a random number between 1 and the number of villages that exist in the raion. Selection of raions followed the same procedure of town selection. Health blocks and villages that were very large in size were divided into segments of approximately households and only one segment was retained for the KDHS. 155

8 A.5 Sampling Probabilities The sampling probabilities were calculated separately for each sampling stage, and independently for each stratum. The following notations were used: P~ is the first-stage sampling probability (counting blocks, towns, or raions). P2 is the second-stage sampling probability (health blocks, villages). P3 is the third-stage sampling probability (households). A.5.1 Almaty City Let a be the number of counting blocks selected and A be the total number of counting blocks in Almaty City. The probability of inclusion of the i 'h counting block in the sample is calculated as follows: a 40 Pli = ~ = 2515 In the second stage, a number, b,, of households was selected from the number M~" of households listed in the ith selected counting block by the KDHS teams. It follows that: P2t- b i M~ / In order for the sample to be self-weighting within the stratum, the overall probability f= PwP2~ must be the same for each household within the stratum. This implies that: bi Pu.P2~ = - f 40M/ wherefis the sampling fraction for Almaty City calculated as follows: f=_n N where n is the number of households selected in Almaty City and N is the estimated number of households that existed in Almaty City in 1995, at the time of fieldwork. A.5.2 Other urban areas First, towns will be discussed. Let a be the number of towns selected in a given stratum M,, the size (population according to the sampling frame) of the i th town in the stratum, and ZM~, the total size of the stratum (population according to the sampling frame). The probability of inclusion of the ith town in the sample is calculated as follows: am~ Pu = ~M~ i 156

9 In the second stage, one health block was selected in each town. The probability of selection of the fh health block in the i th town is as follows: mtj P2q = Emq ] where mii is the size of thej th health block. An intermediary sampling stage was introduced between the second and third sampling stages. This selection stage was not considered an effective stage but only a pseudo-stage in order to reduce the size of the health block. Let t~jk be the estimated size (in proportion) of the k th segment selected for the fh health block. Note that Ytii k = 1. The sampling probabilities are: am~ mqtqk Pn'P2ii = EM~" Emo i 1 In the third stage, a number, b~, of households was selected from the number M~' of households listed in the k ~h segment of thef h health block by the KDHS teams. It follows that: am i motiy k b~ Pn'P2o'P3iJk = Y~M i" ~mo " M/ i J In order for the sample to be self-weighting within the stratum the overall probability f= Pn.Pzo.Pa,ik must be the same for each household within the stratum, wherefis the sampling fraction calculated as in Almaty City, separately for each stratum. The selection of the households was systematic with equal probability and the selection interval was calculated as follows: li 1 Pli'P2ij f P3ijk In the case of self-representing cities, P~ = 1. If more than one health block were selected then: P20 _ a/mq ~m ij where a' is the number of health blocks selected in the city. The other parameters were calculated as those for towns. A.5.3 Rural areas The calculations of the selection probabilities for the different stages of sampling were the same as for the towns, with raions equivalent to towns, and villages equivalent to health blocks. 157

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