Create a Random Sample - Stratified Sampling Tab

Select the Stratified Sampling tab of the Create a Random Sample dialog box to access options to extract a stratified random sample from the current input data file. Multiple stratification variables can be specified, which should contain integer code values to identify the respective strata or groups. The stratified sample will be drawn from the combinations of all (selected) codes for all stratification variables. For example, if you specify two stratification variables Gender : Male/Female and US Citizenship : Yes/No, then different (or identical) sampling fractions can be specified for each combination Male/Yes, Male/No, Female/Yes, Female/No. Note that you can also select specific codes for each stratification variable.

Strata Variables. Select one or more stratification variables. The stratified sample will be drawn from the combinations of all codes for all stratification variables.

Stratification Groups. In the % column, specify the sampling fraction to sample from each stratum; use the respective option (Calculate based on percentage of cases or Calculate based on count of cases) on the Options tab to select either method for determining the sampling fractions. You can also select the Uniform probability check box (see below), in which case the same sampling fraction will be applied to all strata.

Codes. Click the Codes button to display the Select codes for stratification variables dialog box, where you specify codes for the Strata variables. By default, all distinct integer values will be used to define the strata for stratified sampling.

Uniform probability/% =/N =. Select the Uniform probability check box to apply identical sampling fractions to all strata; then specify either the common (to all strata) percentage of cases to be used when drawing the samples, or the approximate numbers of cases; use the respective option (Calculate based on percentage of cases or Calculate based on approximate N) on the Options tab to select either method for determining the sampling fractions. Note that if sample sizes (N) are requested that are greater than the actual number of cases belonging to some strata in the population (in the input file), all cases from those strata will be selected into the final sample.

Count N. When you click this button, the stratification groups are counted for the number of occurrences and the count is placed in the N column. This value is used for balanced sampling.

Exact. Select this check box to ensure that the exact N or exact % of cases specified are returned. Oversampling enables you to specify more cases to be returned than exist in the input. If the number of cases is 50, you can specify 75 or 150% cases back from sampling.