3

I have three groups with ~ 15 individuals per group.

  • Group 1 gets a treatment

  • Group 2 is a control in the same area

  • Group 3 is a further control in a different area

The response is binary, 1 or 0.

I want to determine the power of my study. I used this calculator but would like a second opinion. Using the terms from that site:

I have:

  • 2 independent study groups
  • A dichotomous endpoint
  • alpha = 0.05
  • power = 80%
  • 50% expected incidence in group 1
  • 10% expected incidence in group 2
  • an enrollment ratio of 2:1 because I lump my controls together (groups 2 and 3)

This gives a sample size of 14 for group 1 and 28 for group 2.

kjetil b halvorsen
  • 63,378
  • 26
  • 142
  • 467
adkane
  • 646
  • 7
  • 18
  • You are using logistic regression? Probable duplicates: https://stats.stackexchange.com/questions/162294/multiple-logistic-regression-power-analysis, https://stats.stackexchange.com/questions/35940/simulation-of-logistic-regression-power-analysis-designed-experiments, https://stats.stackexchange.com/questions/26016/sample-size-for-logistic-regression – kjetil b halvorsen Nov 27 '19 at 12:58
  • Why do you lump your controls together in the sample size calculations? That should make for an easier problem, so I guess it will underestimate the needed sample size! You also need to specify how do you plan to do the test. – kjetil b halvorsen Nov 27 '19 at 13:02
  • I group them because neither control gets affected by the treatment. I was thinking of using an Exact test or similar because I am limited by my sample size. – adkane Nov 27 '19 at 13:05
  • Well, but if the base rate depends on area then that should be accounted for in the analysis – kjetil b halvorsen Nov 27 '19 at 13:07
  • There are many posts here about power analysis with exact tests: https://stats.stackexchange.com/search?q=power+exact+test+answers%3A1 – kjetil b halvorsen Nov 27 '19 at 13:16
  • Base rates shouldn't depend on area in my case so I should be ok there. Thanks for the exact test link. – adkane Nov 27 '19 at 14:11
  • Also of interest: https://stats.stackexchange.com/questions/259635/what-is-the-difference-using-a-fishers-exact-test-vs-a-logistic-regression-for, https://stats.stackexchange.com/questions/205904/t-test-chi-squared-or-logistic-regression, https://stats.stackexchange.com/questions/94016/choosing-between-chi-squared-logistic-regression-vs-difference-of-mean-tests-f, https://stats.stackexchange.com/questions/125985/why-do-p-values-for-test-of-likelihood-ratio-vs-fishers-exact-test-not-agree – kjetil b halvorsen Nov 27 '19 at 14:59

0 Answers0