PROC SURVEYSELECT may produce an invalid sample when the options METHOD=PPS_SAMPFORD and CERTSIZE=P= are specified. The sample is invalid when the number of observations selected is less than the requested sample size. The problem does not occur when the CERTSIZE=certain option is specified.

To circumvent this problem, sort the input data set by the SIZE variable or by the SIZE variable within strata. Alternatively, use METHOD=PPS instead of METHOD=PPS_SAMPFORD.