18.1 A metallurgist has designed a way of increasing the strength of steel.

She has discovered a chemical that is added to samples of molten metal during the manufacturing process that have already been measured for strength. These pre-additive values are recorded as the variable called “preadd” in the data table below. She believes that the

“preadd” values may influence the “postadd” measure of the steel’s strength. She is looking for significant differences in strength for the four different manufacturing methods. If differences are found, she wishes to identify which ones contribute to the overall significance.

Can you help her select the correct statistical procedure? She also needs help in writing the null and alternative hypotheses.