4.4 Statistics Notes4.4 Statistics Notes

What are Other Ways to Conduct What are Other Ways to Conduct Experimental and Observational Experimental and Observational

Studies?Studies?

Cluster Random SampleCluster Random Sample

There are ways of sampling other There are ways of sampling other than simple random samplingthan simple random sampling

Cluster random samplingCluster random sampling• First divide the population into large First divide the population into large

groups called clusters, such as towns or groups called clusters, such as towns or blocksblocks

• Then take a simple random sample of Then take a simple random sample of the clusters and sample all subjects in the clusters and sample all subjects in the selected clustersthe selected clusters

Example: PoliceExample: Police

Stratified Random SampleStratified Random Sample

Stratified random samplingStratified random sampling• First divide the population into separate First divide the population into separate

groups called stratagroups called strata• Then take a simple random sample from Then take a simple random sample from

each strataeach strata Example:Example:

Systematic Random SampleSystematic Random Sample

Have a systematic procedure for Have a systematic procedure for collecting a sample of subjectscollecting a sample of subjects• Ex: Sampling every 10Ex: Sampling every 10thth person that person that

walks into your storewalks into your store• Ex: PhonebookEx: Phonebook

Multi-stage Random SampleMulti-stage Random Sample

Taking repeated selections of SRS’s Taking repeated selections of SRS’s within prior SRS’s.within prior SRS’s.• Ex: In order to sample students in the Ex: In order to sample students in the

school, randomly select 5 homerooms, school, randomly select 5 homerooms, then randomly select 10 students within then randomly select 10 students within each of the selected homerooms.each of the selected homerooms.

Advantages and Disadvantages of Advantages and Disadvantages of Sampling MethodsSampling Methods

MethodMethod AdvantagesAdvantages DisadvantagesDisadvantages

Simple Random Simple Random Sample (SRS)Sample (SRS)

Easiest to perform and Easiest to perform and analyzeanalyze

Must have a reliable and Must have a reliable and complete sampling frame of complete sampling frame of

the populationthe population

Cluster random Cluster random samplesample

A reliable sampling frame is A reliable sampling frame is not necessarynot necessary

Less expensiveLess expensive

A larger sample size is needed A larger sample size is needed to achieve the same margin of to achieve the same margin of

error as a SRSerror as a SRS

Stratified Stratified random samplerandom sample

Guaranteed to include a Guaranteed to include a given number of subjects given number of subjects

from each stratumfrom each stratum

Must have a reliable and Must have a reliable and complete sampling frame of complete sampling frame of

each stratumeach stratum

Systematic Systematic samplesample

Must make sure that the Must make sure that the ordering is not connected to ordering is not connected to

the variable of interestthe variable of interest

Multi-stage Multi-stage random samplerandom sample

Types of Observational StudiesTypes of Observational Studies

Retrospective studyRetrospective study – looks into – looks into the pastthe past• Most medical studies are retrospectiveMost medical studies are retrospective

Prospective studyProspective study – follows – follows subjects in the futuresubjects in the future