The article “Advances in Oxygen Equivalence Equations for Predicting the Properties of Titanium Welds”
(D. Harwig, W. Ittiwattana, and H. Castner, The Welding Journal, 2001:126s–136s) reports an experiment to predict various properties of titanium welds. Among other properties, the elongation (in %) was measured, along with the oxygen content and nitrogen content (both in percent). The following MINITAB output presents results of fitting the model
Elongation = β0 + β1 Oxygen + β2 Nitrogen + β3 Oxygen · Nitrogen
a. Predict the elongation for a weld with an oxygen content of 0.15% and a nitrogen content of 0.01%.
b. If two welds both have a nitrogen content of 0.006%, and their oxygen content differs by 0.05%, what would you predict their difference in elongation to be?
c. Two welds have identical oxygen contents, and nitrogen contents that differ by 0.005%. Is this enough information to predict their difference in elongation? If so, predict the elongation. If not, explain what additional information is needed.