A quick guide to sample sizes
Dunlap and Myers (1997) suggest rules of thumb for sample sizes using Cohen's effect size rules of thumb and equations that they have derived in some simple cases. These are given in the table. The sample sizes guarantee at least 80% power to detect the given effect sizes.
Effect Size |
Formula |
Small |
Medium |
Large |
|||
Cohen's d |
unpaired t, equal n |
0.2 |
0.5 |
0.8 |
|||
16/$$d^text{2}$$ + 2 |
|
402 |
66 |
28 |
|||
Correlation |
|
0.1 |
0.3 |
0.5 |
|||
|
8/$$r^text{2}$$ |
800 |
88 |
32 |
|||
$$\phi$$, 2x2 table |
8/$$\phi^text{2}$ |
0.1 |
0.3 |
0.5 |
|||
|
|
800 |
88 |
32 |
|||
$$\phi$$ = square root of (chisquare/sample size)
Reference
Dunlap WP, Myers L. (1997) Approximating power for significance tests with one degree of freedom. Psychological methods 2(2) 186-191.