Other Sampling Methods Lecture 7 Sections 2.6 – 2.8 Wed, Sep 8, 2004

Stratified Random Sampling Stratified random sample – A sample selected by First, dividing the population into mutually exclusive groups, or strata, Then, taking a simple random sample from each stratum.

Why Stratified Samples? We may be genuinely interested in the differences between the strata. By taking samples from each stratum, we can measure those differences. For example, pollsters studying elections routinely stratify their samples by gender and ethnic group.

Why Stratified Samples? It is often the case that the variability within a stratum is much less than the variability between strata. If that is so, then we can get a much better estimate of the population parameter than if we took one random sample from the entire population. Note: The “margin of error” is based on the variability.

Let’s Do It! Let’s do it! 2.7 – Stratified Random Sampling. Substitute “freshman” for “female” and “upperclassman” for “male.” Substitute “average number calls to parents per month” for “average number of haircuts per year.”

Systematic Sampling 1-in-k systematic sampling Label the members of the population 1 through N. Select one member at random from the first k. Select every k-th member from there on.

Example Example 2.18, p. 98 – A 1-in-4 Systematic Sample. Think about it, p. 99.

Cluster Sampling Cluster Sampling – The units of the population are clustered into groups. One or more clusters are selected. All members of the selected clusters are in the sample. Note that it is the clusters that are selected at random, not the individuals.

Let’s Do It! Let’s do it! 2.11, p. 103 – Cluster Sampling of Census Tract Blocks.

Stratified Sampling vs. Cluster Sampling In stratified sampling The members of a stratum (should) have some characteristic in common (homogeneous). From all of the strata we take randomly selected individuals. In cluster sampling The members of a cluster are intended to resemble the entire population (heterogeneous). From randomly selected clusters we take all of the individuals.

Let’s Do It! Let’s do it! 2.12, p. 104 – Cluster Sampling of Students. Let’s do it! 2.13, p. 105 – Which Sampling Method Could Have Been Used?