MEPX
Chapter 5 of 10All chapters
  1. 1What is one row
  2. 2Rates and denominators
  3. 3Beyond the average
  4. 4Compared with what
  5. 5Aggregation traps
  6. 6Noise and significance
  7. 7Metrics and proxies
  8. 8Missing data
  9. 9Reading forecasts
  10. 10A reading routine

Chapter 5 of 10

Aggregation traps

Totals that reverse.

Simpson's paradox

Every subgroup can move one way while the total moves the other, because the mix of subgroups changed. Both figures are correct and only one answers the question.

  • Always break a surprising total down by an obvious grouping.
  • Mix shifts explain many apparently mysterious business results.

Weighted averages

An average of averages is not the overall average unless the groups are equal in size. It is one of the most common spreadsheet errors in circulation.