DATA ABC; 
INPUT TDATE DATE9. AVG; 
CARDS;
18APR2008 150.00
19APR2008 167.00 
20APR2008 123.00
21APR2008 145.00
; 
RUN; 

data _null_;
set abc;
if tdate='18APR2008'D then 
call symput('x',avg);
if tdate='21APR2008'D then 
call symput('y',avg);
z=&x-&y;
run;

data _null_;
z=&x-&y;
put 'the difference='z;
run;

Hi, pls modify slight changes to the above prog.

data _null_ ;
set abc;
if tdate='18APR2008'D then 
call symput('x',avg);
if tdate='21APR2008'D then 
call symput('y',avg);
run;
%put _user_;
data _null_;
z=&x-&y;
put 'the difference=' z;
run;

The Above can be done in a single data step as 
follows.Actually this is suggestable.


data _null_;
set abc;
retain z;
if tdate='18APR2008'D then
z=avg;
if tdate='21APR2008'D then
do;
diff=Z-avg;
put 'the difference=' diff;
stop;
end;
run;

Hi, 
I am think that the difference between the first and last 
records value?

data _null_;
set abc end=last;
retain first;
if _n_=1 then first=avg;
if last=1 then do;
diff=first - avg;
put 'The difference is =' diff;
stop;
end;
run;

here _n_=1 is hold the first record in the abc dataset.
Last=1 means is hold the last record in the dataset.

We can solve this with array......