Re: Effect size in meta-analysis

On Jul 20, 4:19 am, James100 <marylan...@xxxxxxxxxxx> wrote:
On Jul 16, 9:40 am, Ray Koopman <koop...@xxxxxx> wrote:
On Jul 15, 9:31 pm, James100 <marylan...@xxxxxxxxxxx> wrote:

SEM = ??? Standard Error of a Mean?
That would make sense only if all the groups were the same size.
What were the n's?

Many thanks for the response. n's were 3 for all groups.

If SEM is the Standard Error of a Mean, and each mean is based on
3 observations, then I assume that SEM was computed as sqrt(MSE/3),
where MSE is the pooled Mean Square for Error from the anova. With
only 3 observations/group, you're probably better off using the pooled
error term, which averages the variances of all six groups, than you
would be if you were to use only the variances of the two groups whose
means you're comparing, unless the true variances of those two groups
differ substantially from the true variances of the four other groups.
Notice that this refers to the true variances, not the sample
variances. Sample variances based on only 3 values are very poor
estimates of the true variances, so even if you had the sample
variance for each group, I would still recommmend that you use the
average of all six. This is a case where you need to be willing to
accept the possibility of some bias in your estimate of the variance
in exchange for an almost certain reduction in the standard error of
that estimate.
Mean(Trt) - Mean(Control)
Anyhow, your effect size estimate is -------------------------.
SEM * sqrt(3)

Dear Ray,

Have send a message to you directly, but don't know if you have
received it? MANY many thanks fot the very usefull help.

James

Yes, I got it. You're quite welcome. Since you have no local support,
feel free to ask here, even for the little things.
.

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