Appendix A. The T-Distribution Table

The t-distribution is a probability distribution with a symmetrical, bell-shaped curve (similar to the standard normal curve), the shape of which is affected by a parameter known as the “degrees of freedom.” We used t-distributions in Chapter 8 of this book to compute confidence intervals. In that usage, the degrees of freedom controlled how far out you had to go (in terms of standard deviations) on the t-distribution curve from the mean to encompass a given percentage of values. The higher the degrees of freedom, the larger the interval on the curve.

Table A-1 gives t-distribution values for various probabilities, with each row representing 1 additional degree of freedom. Those values in the column for 0.05 (95%) were used in Chapter 8.

Table A-1. The t-distribution table referenced in Chapter 8

DF

Probabilities

0.2

0.1

0.05

0.02

0.01

0.002

0.001

1

3.078

6.314

12.706

31.82

63.66

318.3

637

2

1.886

2.92

4.303

6.965

9.925

22.33

31.6

3

1.638

2.353

3.182

4.541

5.841

10.21

12.92

4

1.533

2.132

2.776

3.747

4.604

7.173

8.61

5

1.476

2.015

2.571

3.365

4.032

5.893

6.869

6

1.44

1.943

2.447

3.143

3.707

5.208

5.959

7

1.415

1.895

2.365

2.998

3.499

4.785

5.408

8

1.397

1.86

2.306

2.896

3.355

4.501

5.041

9

1.383

1.833

2.262

2.821

3.25

4.297

4.781

10

1.372

1.812

2.228

2.764

3.169

4.144

4.587

11

1.363

1.796

2.201

2.718

3.106

4.025

4.437

12

1.356

1.782

2.179

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