Misleading Aggregates:
Simpson's Paradox
Jerry Johnson
Universtity of Nevada
The verb to aggregate means "to collect or gather into a mass or whole." If
you lump data together (aggregate it) you may get one picture, whereas if you break the
data down into categories, you may get a very different picture - sometimes a seemingly
contradictory one. The phenomenon is called "Simpson's Paradox." The examples in this module
illustrate how this can happen and why one should be cautious before drawing
conclusions from numbers. The mathematics involved is arithmetic and percentages.
The mathematics involved is arithmetic and percentages. Suggested grade levels:
12 and up due to subject matter and reading levels.