Assume that we gather independent random samples from large batches of each of three different brands of lightbulbs. We then record the lifetime of each selected bulb in the file P12_9.XLS. 

a. Test whether the different brands of lightbulbs have equal average lifetimes at the 10% significance level. 

b. Compute 90% confidence intervals for all pairs of differences between means. Which of these differences, if any, are significantly nonzero at the 10% significance level?