A farmer wants to know the effect of using various amounts of fertilizer in relation to the yield of his crops. He subdivided a portion of his land into 10 separate, but essentially equal, plots. He divided the land so that all of the plots had equal exposure to sun, rain, and other environmental conditions. He randomly selected 2 of the 10 plots and spread 2 pounds of fertilizer over each of them. Then he randomly selected 2 of the remaining plots and spread 3 pounds of fertilizer over each of them. He continued with the last sets of plots in the same manner, spreading 4, then 5, then 6 pounds of fertilizer over each of them. Assume the conditions for regression inference have been satisfied. To construct the 90% confidence interval for the true slope of the population regression equation relating amount of fertilizer to yield. Required:
How many degrees of freedom does the t-distribution have?