I am running an experiment and I would like what is a good method to calculate how many times I need to run an experiment, in other words, how many data points I will need to collect. The issue is that I do not know my population. Theoretically speaking, I could repeat the measurement many times (assuming no budget or time/budget or other constraints). On the other hand, I know the confidence interval that I'd like to have (95%) and a margin of error of no more than 5%

Assume no prior knowledge about the mean or standard deviation (essentially I will be running this experiment for the 1st time).

Any ideas or suggestions?

  • $\begingroup$ Confidence interval for what? $\endgroup$
    – Dave
    Dec 13, 2019 at 3:03
  • $\begingroup$ For the statistic of interest (e.g. mean) for a metric. $\endgroup$
    – gplt
    Dec 13, 2019 at 7:27
  • $\begingroup$ It may be worth your time to look into power. The statistical power of a hypothesis test is, among other things, related to the number data points (number of subjects in the analysis, for instance). $\endgroup$ Dec 14, 2019 at 5:49
  • $\begingroup$ Hi Adam, I thought about this but I am not dealing with hypotheses in my case. I am dealing with a continuous variable. $\endgroup$
    – gplt
    Dec 16, 2019 at 19:37

1 Answer 1


It appears you want to know how many samples you need to estimate a mean to a given precision with a given confidence but that you don't have a hypothesis to test.

To answer this, you need to do a power analysis. This requires an estimate of the standard error, which you can get from N and the standard deviation. But you need to estimate the SD. You can base this estimate on prior knowledge, a literature review or whatever, but you need to estimate it somehow.

  • $\begingroup$ Well, it depends on how you define power analysis. The ideas are similar. You need to estimate sd. $\endgroup$
    – Peter Flom
    Dec 16, 2019 at 20:04
  • 1
    $\begingroup$ Hi Peter, yes, that is correct. I looked into power analysis but this approach requires having some sort of hypothesis which I don't have (and want to avoid if possible). What I think is right for my case is the following formula for the margin of error: $$ \text{margin of error} = z \frac{\sigma}{\sqrt{n}} $$ where z is the z-score or standard score and which I solve for n and assuming a SD from prior knowledge. Happy to know if there are any alternative or better ways! $\endgroup$
    – gplt
    Dec 16, 2019 at 20:08

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