Sample Size for Unpaired t Test

 

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).

 

Information required

• 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.

 

Practical issues

• 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.

 

Technical validation

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 t_v,p is a Student t quantile with v degrees of freedom and probability p. n is rounded up to the closest integer.