Well you question seems pretty straightforward. It sounds like you only need a confidence interval (not necessarily a prediction interval) and you're just supposed to compare the width for when you use the average values for everything else compared to using the max values for everything else. So instead of changing the interval type from confidence to prediction you should change the other covariates from their average values to the maximal values...
It sounds like the idea is to get you to explore the fact that predictions near the mean values for the covariates have smaller confidence intervals than predictions far away from the mean.
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