Sample size
Ten thousand events from two thousand users is not n = 10,000 for a question about users. The unit you sampled decides what n is.
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Statistical Power
Power is not a property of a test but of a test plus an effect size you name. Without naming one, “underpowered” has no meaning.
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Power Analysis
Sample size, effect size, α and power are four numbers with one equation. Fix any three and the fourth follows — which is the whole method.