How many participants do I need?
It depends on the effect size you want to detect, your alpha, your target power and the test you plan to run. For a between-groups comparison looking for a medium effect at 80% power, the number is often around 60 to 70 per group, but quoting that as a general answer would be misleading. The calculation takes minutes once the inputs are decided, and deciding the inputs is the actual work.
This comes up on Sample Size and Statistical Power, where it is answered in the context of the work itself.
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