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 below 11.2 10.7 12.3 15.9 10.8 12.8 16.9 10.1 11.3 11.0 10.7 12.0 (a) Compute a point estimate of σ, the population standard deviation of main stem diameter. (Round your answer to three decimal places.) σ = Inches What statistic did you use to obtain your estimate? O sample range sample proportion sample standard deviation sample variance sample mean (b) Suppose that the diameter distribution is normal. Then the 90th percentile of the diameter distribution is μ + 1.280 (so 90% of all trees have diameters less than this value). Compute a point estimate for this percentile. (Hint: First compute an estimate of μ and then use it along with your estimate of a from part (a). Round your answer to three decir places.) +1.280 = Inches