The TTEST Procedure

specifies that confidence intervals (except test-based mean confidence intervals when you specify the TOST option) are to be 100(1 – p)% confidence intervals, where 0 < p < 1. When you specify the TOST option, the test-based mean confidence intervals are 100(1 – 2p)% confidence intervals. For the PLOTS=BOOTSTRAP(CORRELATION) option, the ALPHA= option specifies that the elliptical prediction region is to be a 100(1 – p)% prediction region. If p is 0 or less, or 1 or more, an error message is printed. By default, ALPHA=0.05.

groups the results by the variables that are specified in the PAIRED or VAR statement. The BYVAR option is enabled by default. Note that this represents a change from previous releases for how the results are grouped with respect to variables and tables. Prior to SAS 9.2, multiple variables were included in each table, similar to the new NOBYVAR option.

specifies whether a confidence interval is displayed for and, if so, what kind. You can specify one or more of the following values:

The values EQUAL and UMPU together request that both types of confidence intervals be displayed. If the value NONE is specified with one or both of the values EQUAL and UMPU, NONE takes precedence. For more information, see the section Two-Independent-Sample Design. By default, CI=EQUAL.

requests the Cochran and Cox (1950) approximation of the probability level for the unequal variances situation. For more information, see the section Two-Independent-Sample Design.

names the SAS data set for the procedure to use. By default, PROC TTEST uses the most recently created SAS data set. The input data set can contain summary statistics of the observations instead of the observations themselves. The number, mean, and standard deviation of the observations are required for each BY group (one sample and paired differences) or for each class within each BY group (two samples). For more information about the DATA= option, see the section Input Data Set of Statistics.

specifies the underlying distribution assumed for the data. You can specify the following values:

By default, DIST=NORMAL, unless TEST=RATIO is specified, in which case the default is DIST=LOGNORMAL.

requests tests against a null value of m, unless the TOST option is used, in which case m is merely used to derive the lower and upper equivalence bounds. For the crossover design, the value m applies for both treatment and period tests. By default, H0=0 when TEST=DIFF (or DIST=NORMAL for a one-sample design) and H0=1 when TEST=RATIO (or DIST=LOGNORMAL for a one-sample design).

includes all variables that are specified in the PAIRED or VAR statement together in each output table. If the NOBYVAR option is not specified, then the BYVAR option is enabled, grouping the results by the PAIRED and VAR variables.

specifies the order in which to sort the levels of the classification variables (which are specified in the CLASS statement) and treatment variables (which are specified in the CROSSOVER= option in the VAR statement).

This option applies to the levels for all classification or treatment variables, except when you use the ORDER=FORMATTED option with numeric classification or treatment variables that have no explicit format. With this option, the levels of such variables are ordered by their internal value.

You can specify the following values:

For FORMATTED and INTERNAL, the sort order is machine-dependent.

For more information about sort order, see the chapter on the SORT procedure in the Base SAS Procedures Guide and the discussion of BY-group processing in the "Grouping Data" section of SAS Programmers Guide: Essentials.

By default, ORDER=MIXED, which corresponds to the ordering in releases previous to SAS 9.2.

controls the plots produced through ODS Graphics. When you specify only one plot-request, you can omit the parentheses around the plot-request. Here are some examples:

plots=none
plots=(histogram boxplot interval qq profiles agreement)
plots(unpack)=summary
plots(showh0)=interval(type=pergroup)
plots=(summary(unpack) interval(type=period))

ODS Graphics must be enabled before plots can be requested. For example:

ods graphics on;

proc ttest plots=all;
   var oxygen;
run;

ods graphics off;

For more information about enabling and disabling ODS Graphics, see the section Enabling and Disabling ODS Graphics in Chapter 24, Statistical Graphics Using ODS.

If ODS Graphics is enabled but you do not specify the PLOTS option, then PROC TTEST produces a default set of plots. ( Note: The graphical results are unavailable if your input data set contains summary statistics rather than observation values.)

For a one-sample design, the default plots are the following:

For a two-independent-sample design, the default plots are the following:

For a paired design, the default plots are the following:

For a crossover design, the default plots are the following:

For more detailed descriptions of plots, see the section Interpreting Graphs.

You can specify the following global-plot-options:

You can specify the following plot-requests:

specifies the number of sides (or tails) and direction of the statistical tests and test-based confidence intervals. The values are interpreted as follows:

By default, SIDES=2.

specifies the test criterion. This option is ignored for one-sample designs. You can specify the following values:

By default, TEST=DIFF, unless you specify DIST=LOGNORMAL, in which case the default is TEST=RATIO.

performs Schuirmann’s TOST equivalence test. The upper equivalence bound must be specified. If TEST=DIFF, then the default value for the lower equivalence bound is upper, where m is the value of the H0= option. If TEST=RATIO, then the default value for lower is m / upper.