General Mathematics · bivariate data · manipulative

The third variable was doing the work

Ice-cream sales and drownings rise together. Colour by temperature — two clusters, each nearly flat. The pooled slope was the lurking variable.

days pooled line

Pooled first · colouring by temperature is the reveal

Lurking variable

Colour by temperature
split hot vs cool days

Hot days sell more ice cream and more people swim. Cool days do neither.

Working