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STRATIFIED SAMPLINGDEFINITION

Strata: groups of members that share common characteristics

Stratified sampling:  the population is divided into subpopulations (strata) and random samples are taken of each stratum

Stratified sample: has the same proportion of members from each stratum as the population does

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EXAMPLE

A population of 600 men and 500 females is going to be sampled. The proposed sample size is 300. If the company hired to sample this population will be using a stratified sampling technique, how many of each sex will be sampled?

Pros Cons

• Unbiased estimates • may reduce sampling variance• Enables standard statistical methods• Precision depends on sample size• More control

• May not provide representative sample

• No exact estimate for variances• No variety

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EQUATION

𝑺𝒂𝒎𝒑𝒍𝒆 𝒔𝒊𝒛𝒆 𝒇𝒐𝒓 𝒆𝒂𝒄𝒉𝒍𝒂𝒚𝒆𝒓=𝒔𝒊𝒛𝒆𝒐𝒇 𝒘𝒉𝒐𝒍𝒆𝒔𝒂𝒎𝒑𝒍𝒆𝒔𝒊𝒛𝒆𝒐𝒇 𝒑𝒐𝒑𝒖𝒍𝒂𝒕𝒊𝒐𝒏

× 𝒔𝒊𝒛𝒆𝒐𝒇 𝒍𝒂𝒚𝒆𝒓

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PROCESS

You want to sample 40 staff:

This tells us that of our sample of 40,

• 50% should be male, full time.

• 10% should be male, part time.

• 5% should be female, full time.

• 35% should be female, part time.

Population:male, full time: 90male, part time: 18female, full time: 9female, part time: 63Total: 180

Sample size:male, full time: 20male, part time: 4female, full time: 2female, part time: 14Total: 40

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USING THE EQUATION

∴ there are 20 male full time workers that need to be selected