topic Re: Calculate a difference from "baseline" defined by multi variables in a loop in SAS Programming

Calculate a difference from "baseline" defined by multi variables in a loop

braverju — Tue, 30 Jan 2018 15:19:08 GMT

Dear Community,

I have the following dataset, that describes N persons who have up to 3 visits and took 3 measures (vars) at each visit.

Person Visit Var1 Var2 Var3

1 1 10 30 1

1 2 5 20 3

2 1 7 15 5

2 2 5 13 5

3 3 5 13 5

3 1 6 12 4

3 2 1 10 7

3 3 2 8 6

I would like to calculate a difference from the first visit for each Var. The resulting dataset should look like this:

Person Visit Var1 Var2 Var3 Diff_1 Diff2 Diff3

1 1 10 30 1 . . .

1 2 5 20 3 .. -5 -10 2

2 1 7 15 5 . . .

2 2 5 13 5 -2 -17 0

3 3 5 13 5 . . .

3 1 6 12 4 1 -1 -1

3 2 1 10 7 -4 -3 2

3 3 2 8 6 -3 -5 1

I created the following code:

-----------------------------------------------------------------

PROC SORT DATA = have;
by Person VISIT;
RUN;

DATA want;
SET have;
by Person VISIT;
RETAIN BL; //Baseline value = value for the first visit

ARRAY ITEM{3} VAR1 - VAR3;
ARRAY ITEM_sinceBL{3} Var1_sinceBL Var2_sinceBL Var3_sinceBL;

DO I=1 TO 3;
if first.Person THEN BL = ITEM(I);
ELSE ITEM_sinceBL(I) = ITEM(I) - BL;
END;

drop BL;
RUN;

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(I use look because in reality I have 18 variables.) But I don't get the right result. My BL seems to have the wrong value.

I would appreciate your help very much

Julia

Re: Calculate a difference from "baseline" defined by multi variables in a loop

gamotte — Tue, 30 Jan 2018 15:27:55 GMT

Hello,

When you're on the baseline row, you set the BL variable three times so that, on the following rows you're only left with the last computed BL value, BL=ITEM(3).
You have to store baseline values in an array.

Re: Calculate a difference from "baseline" defined by multi variables in a loop

RW9 — Tue, 30 Jan 2018 15:34:14 GMT

DONT CODE ALL IN UPPERCASE!

It hurts the eyes.

So what you want to do is:

get baseline record, merge this onto your data, then calculate.

data want;
  merge have 
        have (where=(visit=1) rename=var1-var_old1 var2=var_old2 var3=var_old3);
  by person;
  array var{3};
  array var_old{3};
  array diff_{3} 8.;
  do i=1 to 3;
    diff_{i}=var{i} - var_old{i};
  end;
run;

Not tested, post test data in the form of a datastep so we have something to work with.

Re: Calculate a difference from "baseline" defined by multi variables in a loop

braverju — Tue, 30 Jan 2018 19:35:01 GMT

Thank you very much! It worked for me.