Other Sampling Methods

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Presentation transcript:

Other Sampling Methods Lecture 8 Sections 2.6 Tue, Sep 4, 2007

Stratified Random Sampling Stratified random sample

Stratified Random Sampling Normally the members within each stratum share a common characteristic that they do not share with members of the other strata. That is, each stratum is homogeneous. Male vs. female. Resident vs. non-resident.

Stratified Random Sampling The population

Stratified Random Sampling The strata The population

Stratified Random Sampling One stratum The population

Stratified Random Sampling One stratum Another stratum The population

Stratified Random Sampling A simple random sample from this stratum The population

Stratified Random Sampling A simple random sample from this stratum A simple random sample from this stratum The population

Stratified Random Sampling Simple random samples from all strata The population

Stratified Random Sampling The stratified random sample The population

Example Let the population be Andrea, Barry, Chantal, Dean, Erin, Felix, Gabrielle, and Humberto. Choose a stratified sample of size n = 4, where the strata are the two sexes. Is the sample representative with regard to sex?

Why Stratified Samples? If we know the proportion of the population that each group comprises, then we increase our chances of getting a representative sample by using a stratified sample.

Strata vs. Populations We may be genuinely interested in the differences among the strata. For example, pollsters studying elections routinely categorize their samples by gender, and ethnic group, party affiliation, etc. However, in that case, the strata are better viewed as distinct populations.