STRATIFIED SAMPLING

presented by

Waiton sherekete

and Tafara mapetese

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STRATIFIED SAMPLING

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1. Stratification: The elements in the population are divided into layers/groups/ strata based on their values on one/several auxiliary variables. The strata must be non-overlapping and together constitute the whole population.

2. Sampling within strata: Samples are selected independently from each stratum. Different selection methods can be used in different strata.

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Eg. Stratification of individuals animals by liveWeight.

Stratum Live weight (KG)

1 170 or less

2 180-240

3 250-340

4 350-440

5 450-540

6 550-640

7 650 or more

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Stratum 1: Northern Sweden

Eg. Regionalstratification

Stratum 2: Mid-Sweden

Stratum 3: Southern Sweden

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Eg. Stratification of individuals animals by live weight and region

Stratum Live weight Region

1 170 or less Northern

2 170 or younger Mid

3 170 or younger Southern

4 180-240 Northern

5 180-240 Mid

6 180-240 Southern

etc. etc. etc.

Advantages

Provides greater precision than a SRS (simple random sample) of the same size

Often requires a smaller sample, which saves money

Can guard against an "unrepresentative" sample Focuses on important subpopulations but ignores

irrelevant ones If measurements within strata have lower

standard deviation, stratification gives smaller error in estimation

Disadvantages

Can be difficult to select relevant stratification variables

Often requires more administrative work than an SRS

Not useful when there are no homogeneous subgroups

Can be expensive

WHY STRATIFY?

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• Gain in precision. If the strata are more homogenous with respect to the study variable(s) than the population as a whole, the precision of the estimates will improve.

• Strata = domains of study. Precision requirements of estimates for certain subpopulations/domains can be assured by using domains as strata.

WHY STRATIFY?, cont’d

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• Practical reasons. For instance nonresponse rates, method of measurement and the quality of auxiliary information may differ between subpopulations, and can be efficiently handled by stratification.

• Administrative reasons. The survey organization may be divided into geographical districts that makes it natural to let each district be a stratum.

Strata size calculation

In general the size of the sample in each stratum is taken in proportion to the size of the stratum. This is called proportional allocation.

Supporse there are 120 cattle in northen region ,180 in mid region and 140 cattle in southern region.Total =440

we are asked to take a sample of 40 cattle in each strata.

The first step is to calculate the percentage of each group of the total.

southern region - 140/440*100=31,8% northen region-12O/440*100=27,2% Mid region =180/440*100=41%

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cont

Then we will calculate the stratum using the percentages provided.

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Thank You

For more info, READ FOR YOURSELF

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