Introduction to Statistics (MAT/SST 115.03 2008S)
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,
that behaves much like that table. However,
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
.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. Of course,
qt expects a second
parameter, which represents the degrees of freedom. Most generally,
For example, to compute t* for a 95% confidence interval with a sample size of 30, we would write
Similarly, to compute t* for a 90% confidence interval with a sample size of 100, we would write
Copyright (c) 2007-8 Samuel A. Rebelsky.
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