Hello all,
I was wondering if anyone could help. Sorry this is a really basic question but I'm not a statistician.
I am trying to work out the power of a paper I'm reading, but to do this I need a standard deviation for the whole sample. The paper presents two separate standard deviations - one for each group in the study.
I don't have access to the raw data, just the mean, SD and N of each group.
These are;
Group 1 mean (mean1) = 18.6
Group 1 SD (SD1) = 26.1
Group 1 N = 20
Group 2 mean (mean2) = 55.7
Group 2 SD (SD2) = 32.1
Group 2 N = 13
So working out the overall mean is easy, it's just ((18.6 x 20) + (55.7 x 13)) / 33
But how do I do the same for the standard deviation? Is it possible?
Thanks in advance,
Brian
You may leave the list at any time by sending the command
SIGNOFF allstat
to [log in to unmask], leaving the subject line blank.
|