An article in Electronic Packaging and Production (2002, Vol. 42) considered the effect of X-ray inspection of integrated circuits. The rads (radiation dose) were studied as a function of current (in milliamps) and exposure time (in minutes).

The data are in Table E12-7.

(a) Fit a multiple linear regression model to these data with rads as the response.

(b) Estimate σ² and the standard errors of the regression coefficients.

(c) Use the model to predict rads when the current is 15 milliamps and the exposure time is 5 seconds.

Exposure Time 0.25 Rads mAmps 7.4 10 14.8 10 0.5 29.6 10 59.2 10 88.8 10 3 296 10 10 444 10 15 592 10 20 11.1 15 0.25 22.2 15 0.5 44.4 15 1 88.8 15 2 133.2 15 444 15 10 666 15 15 888 15 20 14.8 20 0.25 29.6 20 0.5 59.2 20 1 118.4 20 2 177.6 20 3 592 20 10 888 20 15 1184 20 20 22.2 30 0.25 44.4 0.5 88.8 30 177.6 30 2 266.4 30 3 888 30 10 1332 30 15 1776 30 20 29.6 40 0.25 59.2 40 0.5 118.4 40 1 236.8 40 2 355.2 40 3 1184 40 10 1776 40 15 2368 40 20