Observed power, and what to do if your editor asks for post-hoc power analyses

This blog post is now included in the paper "Sample size justification" available at PsyArXiv. Observed power (or post-hoc power) is the statistical power of the test you have performed, based on the effect size estimate from your data. Statistical power is the probability of finding a statistical difference from 0 in your test (aka a ‘significant effect’), if there is a true difference to be found.