Confidence Limits

If you specify the CL option in the TABLES statement, PROC SURVEYFREQ computes confidence limits for the proportions in the frequency and crosstabulation tables. The confidence coefficient is determined according to the value of the ALPHA= option, which by default equals 0.05 and produces 95% confidence limits.

For the proportion in table cell , the confidence limits are computed as

where is the estimate of the proportion in table cell , is the standard error of the estimate, and is the th percentile of the t distribution with df degrees of freedom calculated as described in the section Degrees of Freedom. The confidence limits for row proportions and column proportions are computed similarly to the confidence limits for table cell proportions.

If you specify the CLWT option in the TABLES statement, PROC SURVEYFREQ computes confidence limits for the weighted frequencies, or totals, in the crosstabulation tables.

For the total in table cell , the confidence limits are computed as

where is the estimate of the total frequency in table cell , is the standard error of the estimate, and is the th percentile of the t distribution with df degrees of freedom calculated as described in the section Degrees of Freedom. The confidence limits for row totals, column totals, and the overall total are computed similarly to the confidence limits for table cell totals.

For each table request, PROC SURVEYFREQ produces a nondisplayed ODS table, "Table Summary," which contains the number of observations, strata, and clusters that are included in the analysis of the requested table. When you request confidence limits, the "Table Summary" data set also contains the degrees of freedom df and the value of used to compute the confidence limits. See Example 83.3 for more information about this output data set.