24.18 Airline flight delays. Here are the numbers of flights on time and delayed for two airlines at five airports during a one-month period. 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.

	Alaska Airlines		America West
	On time	Delayed	On time	Delayed
Los Angeles	497	62	694	117
Phoenix	221	12	4840	415
San Diego	212	20	383	65
San Francisco	503	102	320	129
Seattle	1841	305	201	61

(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.)