The book by Davies (Design and Analysis of Industrial Experiments) describes an experiment to study the yield of isatin. The factors studied and their levels are as follows:

Factor Low (−) High (+)
A: Acid strength (%) 87 93 B: Reaction time (min) 15 30 C: Amount of acid (mL) 35 45 D: Reaction temperature (∘C) 60 70 The data from the 24 factorial are shown in Table P6.11.

(a) Fit a main-effects-only model to the data from this experiment. Are any of the main effects significant?

(b) Analyze the residuals. Are there any indications of model inadequacy or violation of the assumptions?

(c) Find an equation for predicting the yield of isatin over the design space. Express the equation in both coded and engineering units.

(d) Is there any indication that adding interactions to the model would improve the results that you have obtained?