The thickness of a plastic film (in mils) on a substrate material is thought to be influenced by the temperature at which the coating is applied. In completely randomized experiment, 11 substrates are coated at 125°F, resulting in a sample mean coating thickness of x̅₁ = 103.5 and a sample standard deviation of s₁ = 10.2. Another 13 substrates are coated at 150°F for which x̅₂ = 99.7 and s₂ = 20.1 are observed. It was originally suspected that raising the process temperature would reduce mean coating thickness.

(a) Do the data support this claim? Use α = 0.01 and assume that the two population standard deviations are not equal. Calculate an approximate P-value for this test.

(b) How could you have answered the question posed regarding the effect of temperature on coating thickness by using a confidence interval? Explain your answer.