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Re: st: Control groups

From   Steven Samuels <>
Subject   Re: st: Control groups
Date   Sun, 11 May 2008 13:03:05 -0400


Without more information, I cannot tell if -samnpsi- or any of the other programs that you tried will give you proper answers. They will be okay, for example, if 1) you measure responses without sampling or by simple random sampling; and 2) you don't intend to generalize your findings beyond the two particular populations you study. Different designs other than aimple two-group cross-sectional comparison might reduce the needed sample size and strengthen your conclusions. Whether your control group is internal (same population) or external (another population) matters, too.

In any case, more details would be helpful.


On May 9, 2008, at 5:52 PM, <> <> wrote:

Dear Statalisters,

Say I have a population of 500 000. I would like to treat this
population with some sort of communication, and to be able to measure
the effect of this treatment I have the opportunity to draw a control
group. Based on earlier experiences the effects between the treated and
the controlgroup could be as small as 0.5%.

I want to be able to detect such a small effects, and I am wondering how
large my control groups ideally would be to track these changes, say at
an alpha level of 0.05.

I have tried to use the fpower (findit fpower) and the simpower (findit
simpower) to determine optimal control group sizes. I am curious whether
there are other alternatives in Stata?

Thanks! And have a nice weekend.

Best wishes,

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