1. 4.4 Statistics Notes What are Other Ways to Conduct Experimental and Observational Studies?

2. Cluster Random Sample There are ways of sampling other than simple random sampling There are ways of sampling other than simple random sampling Cluster random sampling Cluster random sampling First divide the population into large groups called clusters, such as towns or blocksFirst divide the population into large groups called clusters, such as towns or blocks Then take a simple random sample of the clusters and sample all subjects in the selected clustersThen take a simple random sample of the clusters and sample all subjects in the selected clusters Example: Police Example: Police

3. Stratified Random Sample Stratified random sampling Stratified random sampling First divide the population into separate groups called strataFirst divide the population into separate groups called strata Then take a simple random sample from each strataThen take a simple random sample from each strata Example: Example:

4. Systematic Random Sample Have a systematic procedure for collecting a sample of subjects Have a systematic procedure for collecting a sample of subjects Ex: Sampling every 10 th person that walks into your storeEx: Sampling every 10 th person that walks into your store Ex: PhonebookEx: Phonebook

5. Multi-stage Random Sample Taking repeated selections of SRS’s within prior SRS’s. Taking repeated selections of SRS’s within prior SRS’s. Ex: In order to sample students in the school, randomly select 5 homerooms, then randomly select 10 students within each of the selected homerooms.Ex: In order to sample students in the school, randomly select 5 homerooms, then randomly select 10 students within each of the selected homerooms.

6. Advantages and Disadvantages of Sampling Methods MethodAdvantagesDisadvantages Simple Random Sample (SRS) Easiest to perform and analyze Must have a reliable and complete sampling frame of the population Cluster random sample A reliable sampling frame is not necessary Less expensive A larger sample size is needed to achieve the same margin of error as a SRS Stratified random sample Guaranteed to include a given number of subjects from each stratum Must have a reliable and complete sampling frame of each stratum Systematic sample Must make sure that the ordering is not connected to the variable of interest Multi-stage random sample

7. Types of Observational Studies Retrospective study – looks into the past Retrospective study – looks into the past Most medical studies are retrospectiveMost medical studies are retrospective Prospective study – follows subjects in the future Prospective study – follows subjects in the future