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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