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In human research, we define our universe and estimate the effect size we expect to observe, then we can draw the a sample of a given size.

How is sample size determined for experiments in the animal research field?

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Do you mean 'determine the sample size', not 'draw/estimate the sample size'? Is this question in the context of power analysis? –  Pavel Jul 15 '13 at 19:10
    
@Pavel I tweaked the question in line with what you are expecting. –  Jeromy Anglim Jul 16 '13 at 3:23
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Truth is, for many experiments, whether with animals or humans, people don't estimate an effect size or conduct power analysis at all. –  Gaël Laurans Jul 16 '13 at 6:58
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Truth is, for many experiments, people collect data and later wonder what they might do with it. –  what Jul 20 '13 at 14:23
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1 Answer

Determining sample size for an animal experiment is no different than in research with human subjects. What you need to know is the effect size, the significance level and the power (which is the probability that the test detects a significant effect assuming that there is one). The tricky part is getting the effect size (for an interesting discussion have a look at this thread). The other two things are chosen by you as the experimenter. An overview paper on power analysis was written by Cohen (1992).

References

Cohen, J. (1992). A power primer. Psychological Bulletin, 112(1), 155. PDF

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