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I'm doing a cross-sectional descriptive study with an estimated sample size being 140. Will it be statistically justifiable if I managed to collect a sample of 200? Could there be any side effect that may not be scientifically acceptable?

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  • $\begingroup$ No, what would be the scientific unacceptable side effect of more data? The only real down side is that its gonna cost money $\endgroup$ – Repmat May 3 '16 at 6:52
  • $\begingroup$ (This doesn't seem to me that it is so unclear that it needs to be closed.) $\endgroup$ – gung May 3 '16 at 15:39
  • $\begingroup$ It is invalid if you notice your results aren't quite significant, so you keep gathering data to get significance. If that isn't what you're doing, I don't see the problem here. $\endgroup$ – gung May 3 '16 at 15:40
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I can think of at least three possible issues with exceeding a planned sample size, but you will have to judge for yourself, which of these apply in your case.

  1. Perhaps most importantly, there might be ethical issues. E.g. if more samples means more animals or humans being involved in an experiment, then this might be problematic, if an ethics committee approved a much smaller sample size.
  2. A key concern from a statistical perspective is that if this is done based on what the collected data looks like, then there is a problem with keeping type I error rate etc. and approaches like group-sequential methods or adaptive designs might be needed. That should ideally be decided up-front. However, that is not a concern, if your approach is "Without looking at the data we are getting we will collect at least 140 samples and more if we can, then we will go and analyse what we have." I would be tempted to clearly say that in my research protocol though.
  3. It might not be entirely necessary and be a waste of money/resources/effort to collect more samples than planned. If there were a very large discrepancy, the result might mean that you demonstrate very clearly that there is some effect/difference (whatever you are looking for) that is so small that it is irrelevant. However, 200 instead of 140 does not sound like a massive difference in terms of power for the effect size you may have powered for (this would not turn 80% power into 99.9%) and is not going to get you an enormous power for a much smaller effect size.
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