Guys, I really need help with this:
I am trying to do a sample size calculation, for comparison of means of three groups as follows:
%let ngroup=34; /*n per group*/
%let sd=15; /*standard deviation*/
%let power=0.80; /*Required power*/
%let m1=20; /*Mean group 1*/
%let m2=30; /*Mean group 2*/
%let m3=30; /*Mean group 3*/
%let alpha=0.05; /*Mean group 3*/
proc power;
onewayanova test = overall
groupmeans = &m1 | &m2 | &m3
stddev = &sd
npergroup = &ngroup
power = .
alpha=α
plot x=n min=1 max=100 npoints=20;
run;
OK so this is telling me that with 34 per group I will get a power of 0.80
Correct me if I am wrong, but this will only tell you how strong your power is to detect overall difference, and not between group differences.
I am also interested in taking into account pairwise comparisons when doing the sample size calculation, I can think of two strategies and I dont know if they are adequate:
1. use Bonferroni correction dropping the alpha to 0.05/ngroup(3)=0.017
or use contrast but i am not sure if contrasts are right and what /how to use them
Could anyone please please try o help me out with these questions?
