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Thread: Sample size calculation

  1. #1
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    Sample size calculation

    Hi all,

    if got the a question concerning sample size calculation:
    When doing a sample size calculation for a randomized clinical trial for comparing to group means I know the following formula:
    n= \frac{r+1}{r} \frac{\sigma^2 (Z_{\beta}+Z_{\alpha/2})^2}{difference^2}

    Now my question: How does the sample size change when I have a non-randomized trial instead of a randomized one? Which formula to use?

    Thanks in advance for your help.

    Best regards

  2. #2
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    Re: Sample size calculation

    statistical test, and hence sample size formulas, don't typically account for random vs. non-random sample selection; can be a good idea to try your calculations for different values to get a sense of how sensitive the required sample size is to your hypothesized values, e.g. see first 3 here http://powerandsamplesize.com/Calculators/
    Last edited by ted00; 02-08-2014 at 03:08 PM.

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