To find out how satisfied its passengers are, an airline left feedback forms in the lounge reserved for members of its frequent-flyer programme. Nine of every ten forms returned rated the service as excellent, and the airline announced that ninety per cent of its passengers are highly satisfied. What is the strongest objection to the announcement?

The forms reached only frequent flyers, who are precisely the passengers most attached to the airline and least likely to be dissatisfied; the many passengers who never enter that lounge had no way to be heard. A figure drawn from such a group cannot be extended to all passengers. Anonymity and opening hours are irrelevant, and repeat forms would be a smaller problem than the exclusion of most passengers altogether.