Objective:

Use ttest to compare the difference of a continous variable between two group.

You need to verify the assumption of two-sample ttest.

Here is the general form of ttest.

PROC TTEST DATA=SAS-data-set;

CLASS variable;

VAR variables;

PAIRED variable1*variable2;

RUN;

CLASS specifies the two-level variable for the analysis. Only one variable is allowed in the CLASS statement. VAR specifies numeric response variables for the analysis. If the VAR statement is not specified, PROC TTEST analyzes all numeric variables in the input data set that are not listed in a CLASS (or BY) statement. PAIRED specifies pairs of numeric response variables from which difference scores (variable1-variable2) are calculated. A one-sample t test is then performed on the difference scores. • If the CLASS statement and PAIRED statement are omitted, PROC TTEST performs a one-sample t test. • When the CLASS statement is present, a two-sample test is performed. • When a PAIRED statement is present instead, a paired t test is performed.

Case1: equal variance:

If the p-value greater than the significant level, it is the test that variance are equal. Then we use the equal variance t test (pooled) results as shown.

Case2: unequal variance:

If the p-value less than the significant level, it is the test that variance are equal. Then we use the equal variance t test (pooled) results as shown.

Here is the sample code for ttest

DATA UNPAIRED ;

INPUT COLOUR \$ RTIME @@ ;

DATALINES ;

GREEN 232.6    RED 232.0

GREEN 257.5    RED 250.5

GREEN 253.1    RED 237.1

GREEN 205.4    RED 201.5

GREEN 226.0    RED 211.1

RUN ;

DATA PAIRED  ;

INPUT   RTIMEG   RTIMER  @@ ;

DIFFRT = RTIMEG  -  RTIMER  ;

DATALINES ;

232.6    232.0

257.5    250.5

253.1    237.1

205.4    201.5

226.0    211.1

RUN ;

PROC SORT DATA = UNPAIRED ; BY COLOUR ; RUN ;

PROC MEANS   DATA = UNPAIRED   N   MEAN   VAR STD   STDERR MAXDEC = 3  ;

BY COLOUR ;

VAR   RTIME ;

RUN ;

PROC    TTEST    DATA = UNPAIRED ;

CLASS   COLOUR ;   VAR   RTIME ;   RUN ;

PROC  TTEST  DATA = PAIRED;  PAIRED  RTIMEG * RTIMER ;  RUN;