The TTEST Procedure

specifies that confidence intervals (except test-based mean confidence intervals when the TOST option is used) are to be % confidence intervals, where . When the TOST option is used, the test-based mean confidence intervals are % confidence intervals. By default, PROC TTEST uses ALPHA=0.05. If is 0 or less, or 1 or more, an error message is printed.

groups the results by the PAIRED or VAR variables. 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. The CI=EQUAL option specifies an equal-tailed confidence interval, and it is the default. The CI=UMPU option specifies an interval based on the uniformly most powerful unbiased test of . The CI=NONE option requests that no confidence interval be displayed for . 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.

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

requests tests against a null value of , unless the TOST option is used, in which case is merely used to derive the lower and upper equivalence bounds. For the crossover design, the value applies for both treatment and period tests. By default, PROC TTEST uses 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 PAIRED or VAR variables 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). The default is ORDER=MIXED, which corresponds to the ordering in releases previous to SAS 9.2.

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.

The ORDER= option can take the following values:

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

For more information about sorting order, see the chapter on the SORT procedure in the Base SAS Procedures Guide and the discussion of BY-group processing in SAS Language Reference: Concepts.

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 requesting plots. 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 21, 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:

• summary plot (histogram with overlaid normal and kernel densities, box plot, and confidence interval band)

• Q-Q plot

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

• summary plot (comparative histograms with overlaid densities and box plots)

• Q-Q plot

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

• summary plot (histogram, densities, box plot, and confidence interval) of the difference or ratio

• Q-Q plot of the difference or ratio

• profiles plot

• agreement plot

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

• comparative histograms with overlaid densities by treatment and period

• comparative box plots by treatment and period

• Q-Q plots by treatment and period

• profiles over treatment plot

• agreement of treatments plot

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

The global plot options include the following:

The plot requests include the following:

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

SIDES=2

(the default) specifies two-sided tests and confidence intervals for means.

SIDES=L

specifies lower one-sided tests, in which the alternative hypothesis indicates a mean less than the null value, and lower one-sided confidence intervals between minus infinity and the upper confidence limit.

SIDES=U

specifies upper one-sided tests, in which the alternative hypothesis indicates a mean greater than the null value, and upper one-sided confidence intervals between the lower confidence limit and infinity.

specifies the test criterion. Use TEST=DIFF to test the difference of means and TEST=RATIO to test the ratio of means. The default is DIFF, unless DIST=LOGNORMAL is specified, in which case the default is RATIO. This option is ignored for one-sample designs.

requests Schuirman’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 is the value of the H0= option. If TEST=RATIO, then the default value for lower is upper.