The data set wash .txt contains data from a study of a test procedure used for automotive paint. The test consists of applying drops of acid onto a painted steel panel at 1 minute intervals for 30 minutes. The panel is then rinsed and later rated by a person on how well the paint withstood the acid. It was thought that over time the ratings of the panels changed due to further degradation in the paint from residual acid on the panel. It was also thought that rinsing the panel with a neutralizing wash after the test might solve this degradation problem. A test of 36 panels (18 with the wash, 18 without) was conducted. The panels were rated initially and then again after being stored for 2 weeks.

(a) Calculate the pairwise differences and make a normal probability plot of them for each test procedure (wash/no wash).

(b) For each of the two test methods test to see if there is a change in ratings over time. Test, using a = 0.05, and explain what your results mean. Do these results support the thoughts of the lab personnel?

(c) Write a report explaining the experiment and its results in your own words.