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I am conducting a quasi experimental study. Both my control and study groups consist of 30 participants. I wasn't able to compute for the effect size etc. beforehand. Please help me to justify that 30 per group is enough mo make my study valid. Thank you.

Belle
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    I think you have to be a bit more specific, as it depends what you're measuring. You could theoretically justify a sample size of 5, if what you're concerned with is the median, as a sample size of 5 is sufficient to build a ~0.9 CI around the median. – chappers May 11 '15 at 03:09
  • Thank you chappers. I am actually trying to determine the significant difference on the post test scores of my control and study group. Please help me. I don't know what to do. :( – Belle May 11 '15 at 03:35
  • In that case you could perform something like a t-test, which is designed to work for things smaller than your one. It was discussed here: http://stats.stackexchange.com/questions/37993/is-there-a-minimum-sample-size-required-for-the-t-test-to-be-valid The key thing is you'll have to check whether your data is reasonable; to make sure whether or not your study is valid, that would probably assess the power of your test to determine if it is useful in a practical sense. – chappers May 11 '15 at 03:45
  • Thank you Chappers. Can I chat you in private? Please do help me more. :( – Belle May 11 '15 at 05:27
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    On current information in the Q. I don't see any basis whatever to justify 30 per group. What was the reason you actually ended up with that number? – Glen_b May 11 '15 at 05:33

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