# R notes for Activity 19-2: Exploring the t-Distribution

While the book does not ask you to use technology in this section, I thought it would be useful for you to learn a bit about how you can use R for working with the t-distribution.

Particularly since the t-distribution table is complicated to use, it is helpful to be able to have R do the computation for us. R provides a procedure, `qt`, that behaves much like that table. However, `qt`, given an area, computes a t-value with that area to the left (rather than to the right, as shown in the table).

For a 95% confidence interval, we call `qt` with .975. (Why .975? Because there's 0.025 to the right, and therefore 0.975 to the left.) More generally, we can average the confidence level and 1. The `qt` function also expects a second parameter, which represents the degrees of freedom. For example, part d asks us to find the value of t* for a 95% confidence interval with a sample size of 10 (9 degrees of freedom). In R, we would write

```qt(.975, 9)
```

Of course, you should make sure that you know how to use the table to find t* and confirm that you get the same answer in both cases.

Samuel A. Rebelsky, rebelsky@grinnell.edu

Copyright (c) 2007-8 Samuel A. Rebelsky.

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