Question: Airline flight delays. Here are the numbers of flights on time and delayed for two airlines at five airports in one month. Overall on-time percentages for each airline are often reported in the news. The airport that flights serve is a lurking variable that can make such reports misleading.
(a) What percentage of all Alaska Airlines flights were delayed? What percentage of all America West flights were delayed? These are the numbers usually reported.
(b) Now find the percentage of delayed flights for Alaska Airlines at each of the five airports. Do the same for America West.
(c) America West does worse at every one of the five airports, yet does better overall. That sounds impossible. Explain carefully, referring to the data, how this can happen. (The weather in Phoenix and Seattle lies behind this example of Simpson's paradox.)