Bicol University
Question
Asked 11 May 2014
What are other ways of calculating estimated sample size besides using effect size?
I will be doing a control group of older people (without intervention) and was thinking of using its effect size to estimate the sample size of my intervention group further down the timeline. But won't the effect size of the control group (without intervention) be smaller and hence, potentially end up with a bigger than needed intervention group sample size estimate? I am not familiar with statistics and this is my rationale only. Would appreciate your suggestions and other considerations.
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University of Giessen
The effect you are actually interested in is the *difference* in response between your intervention group and your control group. It looks like a two-factorial design (one factor is the "time" (t0, t1, maybe more time points, maybe categorical or metric) and the other factor is "intervention" (binary: yes/no). The specific intervention effect is termed "interaction" of the two factors. For this you should define a relevant size and calculate the required sample size.
Oakland University
If you are only looking at 2 groups and only care about the effect of intervention (add don't care about all the other important factors) you will use something like a t-test. Something I found interesting for dividing up the total number of participants, 'N' is to use the equation, n1 = N*S1/(S1+S2). If you know the standard deviations of each group, S1 and S2, the formula above will give you the most power in your test. I would suggest that you take all the info you can get from each person and use it in your analysis.
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