Stratified Sampling Types PDF

Stratified Sampling Types Garrett Glasgow University of California, Santa Barbara, California, USA Glossary What is Stratified Random Sampling...

Stratified Sampling Types Garrett Glasgow University of California, Santa Barbara, California, USA Glossary What is Stratified Random Sampling? disproportional allocation The allocation of a sample to strata in a way that does not reflect the actual proportion Introduction to Stratified of the strata in the population; also known as over- Random Sampling sampling. optimal allocation The allocation of a sample to strata in As with any other type of sampling, stratified random sam- a way that minimizes the variance of estimated popula- pling is a method by which some observations are drawn tion parameters; in some cases known as Neyman from a population in order to make inferences about the allocation. population as a whole. Simple random sampling poststratification The process of allocating the sample to accomplishes this by giving each sampling unit in the pop- strata after the sample has been drawn. proportional allocation The allocation of a sample to strata ulation an equal probability of being drawn, and then in proportion to the actual proportion of the strata in the drawing a sample of the appropriate size. Stratified random population. sampling begins by dividing the sampling units into a set of quota sampling A nonrandom sampling technique for mutually exclusive and collectively exhaustive groups. allocating the sample to strata. These subpopulations are known as strata. Once the strata strata A set of mutually exclusive and collectively exhaustive have been determined, a simple random sample is indepen- subpopulations of some population; one such unit is dently drawn from each, and the resulting subsamples are a stratum. weighted and combined to form a stratified random sample. stratum weight The proportion of the population a stratum For example, it may make sense to use stratified ran- represents. dom sampling in a survey of individual opinions on tax expenditures on law enforcement, and stratify the sam- pling units into urban and rural households. It might be expected that individuals in relatively high-crime urban Stratified random sampling is the process of dividing the areas will have different opinions on the proper level of tax sampling units within a population into a set of mutually expenditures on policing than residents of rural areas. In exclusive and collectively exhaustive groups, known as this case, stratification would lead to more precise strata. Simple random samples are then drawn from estimates of public opinion on this issue. Populations each strata and combined to form a stratified random can of course be stratified by variables other than geo- sample. Because strata are usually selected to be more graphic location (such as gender or age), and populations homogeneous than the population as a whole, stratifi- can be stratified by multiple variables. cation can lead to large improvements in the precision of estimated parameters. Stratified sampling is also used when one or more strata in the population are Why Stratified Random Sampling? relatively rare, and an oversample of this subpopulation There are a number of reasons why a researcher may opt is desired. for a stratified random sample. First, stratification may Encyclopedia of Social Measurement, Volume 3 Ó2005, Elsevier Inc. All Rights Reserved. 683