Smoking and survival

A naive examination suggests that smokers are less likely to die — the proportion dying between the two surveys was higher for non-smokers than for smokers.

However click Slice and drag the slider. Within any age group the proportion of smokers dying is higher than that of non-smokers.

The direction of the marginal relationship was caused by a lurking variable, Age.

Older women were much more likely to die in the 20 years than young women. They were also much less likely to be smokers.

The smokers were less likely to die because they were younger.


In 1972-74, a one-in-six health survey was conducted in Whickham in the North of England. Twenty years later, a follow-up study was conducted. Data were collected about whether each of the 1,314 women was a current smoker in the earlier survey and whether she survived until the second survey.

Other variables such as the ages of the women were also recorded.