The following data represent more extended measurements of monthly water usage at the plant referred to in Exercise 98 over a period of 20 months:
(a) Use an appropriate computer program to fit a linear surface to these data.
(b) Use a computer program to make a normal-scores plot of the residuals. Does the assumption of normality appear to be satisfied at least approximately?
(c) Plot the residuals against 
and determine whether the pattern is random.
(d) Check for excessive multi collinearity among the independent variables.
Exercise 98
(a) Use an appropriate computer program to fit a plane to the following data relating the monthly water usage of a production plant (gallons) to its monthly production (tons), mean monthly ambient temperature (?F), and the monthly number of days of plant operation over a period of 12 months.
(b) Estimate the water usage of the plant during a month when its production is 90.0 tons, the mean ambient temperature is 65?F, and it operates for 20 days.