Testing Difference in Mean Between Two Specific Periods

[arii]

Hi,

I am trying to test if the mean of two variables differ within two specific periods.

Precisely, I want to test if the mean of RSP differs in the period: 25Jan2002 through 24Feb2003, compared to the period: 25Feb2003 through 26Mar2004.

I tried to create a 'flag' variable to distinguish both time periods. However, my log files suggests that the 'CLASS variable does not have two levels'. I am not too sure why the class flag variable does not have two levels as I have enough observations within the specified periods.

See my codes:

data xx;
set x;

Startdate = '25Jan2002'd < '24Feb2003'd < '24Feb2003'd;
Enddate = '25Feb2003'd < '24Feb2003'd < '26Mar2004'd;

If Startdate <= '24Feb2003'd <= Enddate then flag=1;
else flag = 0;
run;

proc ttest data=xx;
class flag;
var RSP;

run;
run;

Log File:

data xx;
set x;
Startdate = '25Jan2002'd < '24Feb2003'd < '24Feb2003'd;
Enddate = '24Feb2003'd < '24Feb2003'd < '26Mar2004'd;

If Startdate <= '24Feb2003'd <= Enddate then flag=1;
else flag = 0;

run;

NOTE: There were 6179 observations read from the data set WORK.SPVIXTWO.
NOTE: The data set WORK.SPVIXTWOX has 6179 observations and 34 variables.

proc ttest data=xx;
class flag;
var VarRSP20;
run;

ERROR: The CLASS variable does not have two levels.

Is there an alternative way to do this or are my codes wrong? I would be glad to provide clarifications where necessary.

Thank you in anticipation!

[PaigeMiller · Accepted solution]

You need to use your date variable to determine the flags, but you haven't done that.

For example if the variable that contains the date in data set x is named DATE, then

If '25jan2002'd<date<'24feb2003'd then flag=1;

[arii]

Thank you!