03RanRegression

Sales from advertising spend

I plotted sales against TV, radio and newspaper budgets, then fitted a linear regression on all three and scored it on one 80/20 split.

What it printed in 2022

Numbers read from the saved notebook outputs. Cell numbers count every cell in the notebook, Markdown included, starting at 1.
Output	Value	Cell #
Test R² (linear regression)	0.906	14
Predicted sales for one budget mix	21.37	15

Cell # counts every cell from the top of the notebook, Markdown included, starting at 1.

R² of 0.906 comes from 40 test rows chosen by one random seed.

A 200-row advertising table, a re-upload of the Advertising data from An Introduction to Statistical Learning with some sales values changed.

Use repeated cross-validation. With 40 test rows the R² moves by more than 0.2 depending on which rows land in the test set.
Report the error in sales units as well as R².
Look at the coefficients. Newspaper spend adds almost nothing once TV and radio are in the model.

03 · 14
Weaker than it looked
One lucky split: R² 0.906 from 40 rows
Across 1,000 random splits of the book's Advertising data, the same model scores between 0.75 and 0.96. Repeated cross-validation settles at 0.886.
Read the audit