Chapter 9.1, Problem 8E

A random sample of n = 12 four-year-old red pine trees was selected, and the diameter (in inches) of each tree’s main stem was measured. The resulting observations are given here:

1. a. Calculate a point estimate of σ， the population standard deviation of main stem diameter. What statistic did you use to obtain your estimate?
2. b. Suppose that the diameter distribution is normal. Then the 90th percentile of the diameter distribution is μ + 1.28σ （so 90% of all trees have diameters less than this value). Calculate a point estimate for this percentile. (Hint: First calculate an estimate of μ and then use it along with your estimate of σ from Part (a).)