11.31 Predicting retail sales.
Daily sales at a secondhand shop are recorded over a 25-day period.8 The daily gross sales and total number of items sold are broken down into items paid by check, cash, and credit card. The owners expect that the daily numbers of cash items, check items, and credit card items sold will accurately predict gross sales.
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11.31
(a) All four variables are somewhat right-skewed. There is a potential outlier for gross sales. (b) All three explanatory variables look linearly related with gross sales but each scatterplot has a few semi-outlying observations that could be potentially influential. From the correlation matrix, we can see that both cash items and check items have quite strong linear relationships with gross sales, but they also have some correlation between them. (c) . (d) The Normal quantile plot shows a roughly Normal distribution with no outliers. The three residual plots all look pretty good (random) but show a couple semi-outlying observations we identified earlier. (e) The intercept is not significantly different from 0; .