A popular effect size for t-tests (unpaired or paired) is Cohen's d which represents the standardized difference between two group means. The highlighted rows in the table below represent small, medium and large effect sizes as suggested by Cohen (1988). This table gives sample sizes required in each of the two groups.
|
90% |
80% |
||||||
d |
1-tailed |
2-tailed |
1-tailed |
2-tailed |
||||
0.1 |
1708 |
2096 |
1234 |
1568 |
||||
0.2 |
428 |
525 |
310 |
393 |
||||
0.3 |
191 |
234 |
138 |
175 |
||||
0.4 |
108 |
133 |
78 |
100 |
||||
0.5 |
70 |
85 |
51 |
64 |
||||
0.6 |
49 |
60 |
36 |
45 |
||||
0.7 |
36 |
45 |
27 |
34 |
||||
0.8 |
28 |
34 |
21 |
26 |
||||
0.9 |
23 |
27 |
17 |
21 |
||||
1.0 |
19 |
23 |
14 |
17 |
||||
1.1 |
16 |
19 |
12 |
15 |
||||
1.2 |
13 |
16 |
10 |
13 |
||||
1.3 |
12 |
14 |
9 |
11 |
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