why the answer is 8 ?

[tianerhu]

data work.numbers;
infile datalines;
input T;
datalines;
6
7
8
;
proc print data = work.numbers noobs;
run;

title 'sum of lag';
data work.move;
  set work.numbers;
  if _N_= 3 then total=sum(T,lag1(T),lag2(T));
run;
proc print data = work.move noobs;
run;

[Kurt_Bremser · Accepted solution]

Because you commit the #1 capital sin of LAG: using the function conditionally. The LAG functions will only populate their queue when actually called, so you put nothing into the queue until the third observation, where the result is still missing.

[tianerhu]

Thank you for your help.

[Tom]

Looks like you want this program instead.

data work.numbers;
  input T;
datalines;
6
7
8
;
data work.move;
  set work.numbers;
  total=sum(T,lag1(T),lag2(T));
  if _N_< 3 then call missing(total);
run;