The SURVEYMEANS Procedure

Degrees of Freedom

PROC SURVEYMEANS computes degrees of freedom df to obtain the $100(1 - \alpha )$ % confidence limits for means, proportions, totals, ratios, and other statistics. The degrees of freedom computation depends on the variance estimation method that you request. Missing values can affect the degrees of freedom computation. See the section Missing Values for details.

Taylor Series Variance Estimation

For the Taylor series method, PROC SURVEYMEANS calculates the degrees of freedom for the t test as the number of clusters minus the number of strata. If there are no clusters, then the degrees of freedom equal the number of observations minus the number of strata. If the design is not stratified, then the degrees of freedom equal the number of PSUs minus one.

If all observations in a stratum are excluded from the analysis due to missing values, then that stratum is called an empty stratum. Empty strata are not counted in the total number of strata for the table. Similarly, empty clusters and missing observations are not included in the total counts of cluster and observations that are used to compute the degrees of freedom for the analysis.

If you specify the MISSING option, missing values are treated as valid nonmissing levels for a categorical variable and are included in computing degrees of freedom. If you specify the NOMCAR option for Taylor series variance estimation, observations with missing values for an analysis variable are included in computing degrees of freedom.

Replicate-Based Variance Estimation

When there is a REPWEIGHTS statement, the degrees of freedom equal the number of REPWEIGHTS variables, unless you specify an alternative in the DF= option in a REPWEIGHTS statement.

For BRR or jackknife variance estimation without a REPWEIGHT statement, by default PROC SURVEYMEANS computes the degrees of freedom by using all valid observations in the input data set. A valid observation is an observation that has a positive value of the WEIGHT variable and nonmissing values of the STRATA and CLUSTER variables unless you specify the MISSING option. See the section Data and Sample Design Summary for details about valid observations.

For BRR variance estimation (including Fay’s method) without a REPWEIGHTS statement, PROC SURVEYMEANS calculates the degrees of freedom as the number of strata. PROC SURVEYMEANS bases the number of strata on all valid observations in the data set, unless you specify the DFADJ method-option for VARMETHOD=BRR. When you specify the DFADJ option, the procedure computes the degrees of freedom as the number of nonmissing strata for an analysis variable. This excludes any empty strata that occur when observations with missing values of that analysis variable are removed.

For jackknife variance estimation without a REPWEIGHTS statement, PROC SURVEYMEANS calculates the degrees of freedom as the number of clusters (or number of observations if there are no clusters) minus the number of strata (or one if there are no strata). For jackknife variance estimation, PROC SURVEYMEANS bases the number of strata and clusters on all valid observations in the data set, unless you specify the DFADJ method-option for VARMETHOD=JACKKNIFE. When you specify the DFADJ option, the procedure computes the degrees of freedom from the number of nonmissing strata and clusters for an analysis variable. This excludes any empty strata or clusters that occur when observations with missing values of an analysis variable are removed.

The procedure displays the degrees of freedom for the t test if you specify the keyword DF in the PROC SURVEYMEANS statement.