9.42 For a certain species of fish, the LC50 measurements

(in parts per million) for DDT in 12 experiments were as follows, according to the EPA:

16, 5, 21, 19, 10, 5, 8, 2, 7, 2, 4, 9 Another common insecticide, Diazinon, gave LC50 measurements of 7.8, 1.6, and 1.3 in three independent experiments.

a Estimate the difference between the mean LC50 for DDT and the mean LC50 for Diazinon in a 90% confidence interval.

b What assumptions are necessary for your answer to be valid?

c Estimate the true variance ratio in a 90%

confidence interval.