Wading Through the Data Swamp:
Program Evaluation 201

Supplements

Critical t-Value Table

This table contains the significant levels for one and two tailed t-tests. (A tail is either end of a normal distribution curve, negative or positive.) To use a one-tailed test we would really need to have a strong reason to believe before the test about the direction our difference would be going, meaning.

Will the t-value be positive or negative? If we have not predicted the difference we should use a two-tailed test. This will protect us if the difference goes either direction. We'll still be able to say something.

The left column of the table contains the degrees of freedom. To use the table:

  1. Locate the degrees of freedom.
  2. Go to the right and find the intersect value at the 0.5 level of significance for a two-tailed test.
  3. The intersect number is the critical value.
  4. To reject the null hypothesis, the calculated t-value in step 10 must be greater than the critical value.
  Level of Significance
 

One-Tailed Test
 

.05

.025
 

Two-Tailed Test

Degrees of Freedom

.10

.05

1

6.314

12.706

2

2.920

4.303

3

2.353

3.182

4

2.132

2.776

5

2.015

2.571

6

1.943

2.447

7

1.895

2.365

8

1.860

2.306

9

1.833

2.262

10

1.812

2.228

11

1.796

2.201

12

1.782

2.179

13

1.771

2.160

14

1.761

2.145

15

1.753

2.131

16

1.746

2.120

17

1.740

2.110

18

1.734

2.101

19

1.729

2.093

20

1.725

2.086

21

1.721

2.080

22

1.717

2.074

23

1.714

2.069

24

1.711

2.064

25

1.708

2.060

26

1.706

2.056

27

1.703

2.052

28

1.701

2.048

29

1.699

2.045

30

1.697

2.042

40

1.684

2.021

60

1.671

2.000

120

1.658

1.980

infinity

1.645

1.960

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