1. Stratified Sampling Lecture 8 Section 2.6 Wed, Jan 28, 2004

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

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

4. Why Stratified Samples? It is often the case that the variability within a stratum is much less than the variability between strata. 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. 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.

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

6. Assignment Page 112: Exercises 25 – 34. Page 112: Exercises 25 – 34.