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Condition 2: Lack of Spuriousness

Just as weak correlations can be causal, strong correlations can turn out not to be. When this happens, the original correlation is said to be spurious. There is a correlation between the number of firefighters at a fire and the amount of damage done: the more firefighters, the higher the insurance claim. You could easily conclude that firefighters cause fire damage.

We know better: Both the amount of damage and the number of firefighters is caused by the size of the blaze. We need to control for this third variable—the size of the blaze—to understand what’s really going on.

Dellino (1984) found an inverse relation between perceived quality of life and involvement with the tourism industry on the island of Exuma in the Bahamas. When he controlled for the size of the community (he studied several on the island), the original correlation disappeared. People in the more congested areas were more likely to score low on the perceived-quality-of-life index whether or not they were involved with tourism, and those in the small, outlying communities were more likely to score high on the index. People in the congested areas were also more likely to be involved in tourism-related activities, because that’s where the tourists go.

Mwango (1986) found that illiterates in Malawi were much more likely than literates to brew beer for sale from part of their maize crop. The covariation vanished when he controlled for wealth, which causes both greater education (hence, literacy) and the purchase, rather than the brewing, of maize beer.

The list of spurious relations is endless, and it is not always easy to detect them for the frauds that they are. A higher percentage of men than women get lung cancer, but when you control for the length of time that people have smoked, the gender difference in lung cancer vanishes. Pretty consistently, young people accept new technologies more readily than older people, but in many societies, the relation between age and readiness to adopt innovations disappears when you control for level of education. Urban migrants from tribal groups often give up polygyny in Africa and Asia, but both migration and abandonment of polygyny are likely to be caused by a third factor: lack of wealth.

Your only defense against spurious covariations is vigilance. No matter how obvious a covariation may appear, discuss it with disinterested colleagues—people who have no stake at all in telling you what you want to hear. Present your initial findings in class seminars at your university or where you work. Beg people to find potentially spurious relations in your work. You’ll thank them for it if they do.

 
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