Systematic Random Sampling

The SURVEYSELECT Procedure

Systematic Random Sampling

The method of systematic random sampling (METHOD=SYS) selects units at a fixed interval throughout the sampling frame or stratum after a random start. PROC SURVEYSELECT chooses the first unit randomly from the entire stratum and then treats the stratum observations as a closed loop. This is done to obtain an unbiased variance estimator, as suggested by Lahiri (Murthy 1967). If you specify the sample size (or the stratum sample sizes) with the SAMPSIZE= option, PROC SURVEYSELECT uses a fractional interval to provide exactly the specified sample size. The interval equals N/n, or N_h/n_h for stratified sampling. The selection probability for each unit equals n/N, or n_h/N_h for stratified sampling. If you specify the sampling rate (or the stratum sampling rates) with the SAMPRATE= option, PROC SURVEYSELECT uses the inverse of the rate as the interval for systematic selection. The selection probability for each unit equals the specified rate.

Systematic random sampling controls the distribution of the sample by spreading it throughout the sampling frame or stratum at equal intervals, thus providing implicit stratification. You can use the CONTROL statement to order the input data set by the CONTROL variables before sample selection. If you also use a STRATA statement, PROC SURVEYSELECT sorts by the CONTROL variables within strata. If you do not specify a CONTROL statement, PROC SURVEYSELECT applies systematic selection to the observations in the order in which they appear in the input data set.

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