Averages look easy, which is why tests use them to set traps.
The big one is the average of averages. If one team of 40 people averages 80 and another team of 10 averages 60, the overall average isn't 70. Bigger groups count for more: (40 × 80 + 10 × 60) ÷ 50 = 76. The same goes for rates. To get an overall conversion rate, divide total purchases by total visits; never average the campaign rates.
Two more tools:
- The median is the middle value once the numbers are sorted. With an even count, take the halfway point between the two middle values.
- For a missing value, work from totals. If 5 numbers average 20, they add up to 100; subtract the ones you know.
Before you calculate, check which way the answer should lean: toward the bigger group.