Math 1 · Unit 10 · Lesson 5 · part 2 of 5

A pattern is not a reason

The last idea in this unit is a warning about what a good fit can and cannot tell you.

A pattern is not a reason

The last idea in this unit is a warning about what a good fit can and cannot tell you. Even a perfect pattern does not prove that one thing causes the other.

AssociationThere is a pattern between two quantities: they tend to move together.

CorrelationHow strong the straight-line pattern is. That is what r measures.

CausationOne thing actually makes the other happen. This is a much bigger claim, and a graph alone cannot prove it.

A famous funny example: for a number of years, the number of letters in the winning word of the national Spelling Bee and the number of people killed by venomous spiders rose and fell together. The points make a convincing scatter plot. But spelling words obviously do not cause spider bites, and spider bites do not make spelling words longer. It is a coincidence: a pattern that happened by chance. Look at enough pairs of things and some will match up by accident.

Association is not causation. Two quantities can move together without either one causing the other. A strong r tells you there is a pattern. It never tells you why.

Where to go next

Next: Lurking variables, and how cause is really shownCoincidence is one reason. All the partsAssociation Is Not Causation