Exercise 5: Regression

Using marketing_mix.csv:

1. Plot sales against each advertising variable.
2. Estimate a model with the three advertising channels.
3. Add discount and competitor price index.
4. Compare adjusted R-squared across models.
5. Interpret the search-advertising coefficient in its original units.
6. Inspect VIF values and diagnostic plots.
7. Identify the observation with the largest Cook’s distance.
8. Re-estimate the model without that case only as a sensitivity check.
9. Explain whether the substantive conclusion changes and why deletion should not be automatic.