Bangladesh 1993-1994 Demographic and Health ... - Measure DHS

APPENDIX BESTIMATES OF SAMPLING ERRORSThe estimates from a sample survey are affected by two types of errors: nonsampling errors andsampling errors. Nonsampling errors are the results of mistakes made in implementing data collection anddata processing, such as failure to locate and interview the correct household, misunderstanding of thequestions on the part of either the interviewer or the respondent, and data entry errors. Although numerousefforts were made during the implementation of the BDHS to minimize this type of error, nonsampling errorsare impossible to avoid and difficult to evaluate statistically.Sampling errors, on the other hand, can be evaluated statistically. The sample of respondents selectedin the BDHS is only one of many samples that could have been selected from the same population, using thesame design and expected size. Each of these samples would yield results that differ somewhat from theresults of the actual sample selected. Sampling errors are a measure of the variability between all possiblesamples. Although the degree of variability is not known exactly, it can be estimated from the survey results.Sampling error is usually measured in terms of the standard error for a particular statistic (mean,percentage, etc.), which is the square root of the variance. The standard error can be used to calculateconfidence intervals within which the true value for the population can reasonably be assumed to fall. Forexample, for any given statistic calculated from a sample survey, the value of that statistic will fall within arange of plus or minus two times the standard error of that statistic in 95 percent of all possible samples ofidentical size and design.If the sample of respondents had been selected as a simple random sample, it would have beenpossible to use straightforward formulas for calculating sampling errors. However, the BDHS sample is theresult of a two-stage stratified design, and, consequently, it was necessary to use more complex formulas.The computer software used to calculate sampling errors for the BDHS is the ISSA Sampling Error Module(ISSAS). This module used the Taylor linearization method of variance estimation for survey estimates thatare means or proportions. The Jacknife repeated replication method is used for variance estimation of morecomplex statistics such as fertility and mortality rates.The Taylor linearization method treats any percentage or average as a ratio estimate, r = y/x, wherey represents the total sample value for variable y, and x represents the total number of cases in the group orsubgroup under consideration. The variance of r is computed using the formula given below, with thestandard error being the square root of the variance:in whichvar,r, h I'; )]= Y a-r'xh , and z h = Yh-r'x hwhere hmhy~.frepresents the stratum which varies from 1 to H,is the total number of enumeration areas selected in the hthstratum,is the sum of the values of variable y in EA i in the h th stratum,is the sum of the number of cases in EA i in the h th stratum, andis the overall sampling fraction, which is so small that it is ignored.151