calculate pooled sigma

[chrisgo]

given dataset below, i want to compute the pooled sigma by Group. Already tried Ttest but it is limited to two class variables

Group	COUNT	MEAN	SIGMA	MEAS_DATE
a	501	101.1664	12.62792	14-Apr-15
a	123	103.33	10.545	8-Apr-15
b	164	99.21071	16.14705	8-Apr-15
b	697	86.24266	9.794989	9-Apr-15
c	223	82.22	7.422	7-Apr-15
c	780	84.10039	8.913545	8-Apr-15

[Reeza]

Assuming sigma is standard deviation and therefore variance I would look into PROC GLM.

PS. Stats questions are best posted under Statistical Procedure forum.

[stat_sas]

data have;

input Group $ COUNT MEAN SIGMA MEAS_DATE :anydtdte.;

format meas_date ddmmyy8.;

datalines;

a 501 101.1664 12.62792 14-Apr-15

a 123 103.33 10.545 8-Apr-15

b 164 99.21071 16.14705 8-Apr-15

b 697 86.24266 9.794989 9-Apr-15

c 223 82.22 7.422 7-Apr-15

c 780 84.10039 8.913545 8-Apr-15

;

proc sql;

select *,sqrt(sum((count-1)*sigma**2)/sum(count)-count(*)) as pool_sigma from have

group by group;

quit;

[Rick_SAS]

Are you missing a set of parentheses?  Seem like it should be

sqrt(sum((count-1)*sigma**2) / (sum(count)-count(*)))

Pooled variance - Wikipedia, the free encyclopedia

[stat_sas · Accepted solution]

Thanks Dr. Rick - Yes, we need one extra set of parentheses to formulate this correctly.

proc sql;

select *,sqrt(sum((count-1)*sigma**2) / (sum(count)-count(*))) as pool_sigma from have

group by group;

quit;