Example Codes: SAS #1 SAS #2 R #1
Objective
Use
t-test to compare the difference of a continuous variable
between two groups. You need to
verify the assumptions of a two-sample
t-test.
Here
is the general form of t-test:
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.
Case 1 - equal variance
If the p-value is greater than the significant level, it is the test that the variance is equal. Then we use the
equal variance t-test
(pooled) results as shown.
Case 2 - unequal
variance
If the p-value less than the significant level, it is the test that
variance is unequal. Then we use the unequal variance t-test (Satterthwaite) results as shown.
Example Code in SAS
proc ttest data=sasuser.TestScores
plots(shownull)=interval;
class Gender;
var SATScore;
run;
Example Code in SAS
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;