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Type the odd letters out: scieNce GATHeRS knowledge fAster tHAN SOCIeTY GATHErS wisdom

location: FAQ / dunp

Sample sizes for testing differences in an unpaired t-test

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