An article in Industrial Quality Control (1956, pp. 5-8) 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 specified brightness level. The data are shown in the following table.

Phosphor Type
Glass Type 1 2 3
280 300 290
1 290 310 285
285 295 290
230 260 220
2 235 240 225
240 235 230

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

(b) Test the above hypotheses and draw conclusions using the analysis of variance with ? = 0.05.

(c) Analyze the residuals from this experiment.