On Jul 16, 9:40=A0am, Ray Koopman <koop...@[EMAIL PROTECTED]
> wrote:
> On Jul 15, 9:31 pm, James100 <marylan...@[EMAIL PROTECTED]
> wrote:
>
> > > SEM =3D ??? =A0Standard 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.
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
=
=A0 Mean(Trt) - Mean(Control)
> Anyhow, your effect size estimate is =A0-------------------------.
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
=
=A0 =A0 =A0 =A0 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
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