42. A more extensive tabulation of t critical values than what appears in this book shows that for the t distribution with 20 df, the areas to the right of the values

.687, .860, and 1.064 are .25, .20, and .15, respectively.

What is the con dence level for each of the following three con dence intervals for the mean m of a normal population distribution? Which of the three intervals would you recommend be used, and why?

a.

b.

c.