An article in Industrial Quality Control (1956) describes an experiment to investigate the effect of two factors (glass type and phosphor type) on the brightness of a television tube. The response variable measured is the current (in microamps) necessary to obtain a specified brightness level. The data are shown in the following table:

(a) State the hypotheses of interest in this experiment.

(b) Test the hypotheses in part (a) and draw conclusions using the analysis of variance with α = 0.05.

(c) Analyze the residuals from this experiment.

Phosphor Type Glass Type 1 2 300 280 290 285 290 310 295 290 285 230 260 220 235 240 225 240 235 230