Menu location: Analysis_Sample Size_Unpaired t.
This function gives you the minimum number of experimental subjects needed to detect a true difference DELTA in population means with power POWER and two sided type I error probability ALPHA (Dupont, 1990; Pearson and Hartley, 1970).
- POWER: probability of detecting a true effect.
- ALPHA: probability of detecting a false effect (two sided: double this if you need one sided).
- DELTA: difference in population means.
- SD: estimated standard deviation for within group differences.
- M: number of control subjects per experimental subject.
- Usual values for POWER are 80%, 85% and 90%; try several in order to explore/scope.
- 5% is the usual choice for ALPHA.
- SD is usually estimated from previous studies.
- If possible, choose a range of differences between means that you want have the statistical power to detect.
The estimated sample size n is calculated as the solution of:
- where d = delta/sd, α = alpha, β = 1 - power, m is the number of control subjects per experimental subject, and tv,p is a Student t quantile with v degrees of freedom and probability p. n is rounded up to the closest integer.