A recent report from the New York Times (subscription) showed how a top-class newspaper clearly distinguishes correlation from causation:
A much-anticipated report from the largest and longest-running study of American child care has found that keeping a preschooler in a day care center for a year or more increased the likelihood that the child would become disruptive in class — and that the effect persisted through the sixth grade.
The effect was slight, and well within the normal range for healthy children, the researchers found. And as expected, parents’ guidance and their genes had by far the strongest influence on how children behaved. …
The research, being reported today as part of the federally financed Study of Early Child Care and Youth Development, tracked more than 1,300 children in various arrangements, including staying home with a parent; being cared for by a nanny or a relative; or attending a large day care center. Once the subjects reached school, the study used teacher ratings of each child to assess behaviors like interrupting class, teasing and bullying. …Â
Others experts were quick to question the results. The researchers could not randomly assign children to one kind of care or another; parents chose the kind of care that suited them. That meant there was no control group, so determining cause and effect was not possible.
By contrast, a report in today’s Sydney Morning Herald shows what happens when you assume that correlation equals causation.
Women will not be allowed to insist on caesarean deliveries in NSW public hospitals without a medical reason under a new health department policy. …
The policy cites a US study of more than 5 million births, which found last year that babies born by medically unnecessary caesarean were three times as likely to die in the newborn period as those born vaginally. The death rate for the caesarean babies was 1.77 for every 1000 live births, compared with 0.62 from normal delivery.
To be fair to SMH journalist Julie Robotham, her report was principally about the policy announcement, not the study. But she should at least have reminded readers that a study that merely looks atÂ correlations cannot tell us whether c-sections are more dangerous – regardless of the sample size. To answer the causal question, we would need to have some random (or quasi-random) assignment of women to c-sections and vaginal births. In the absence of that kind of study, the question of whether c-sections are more dangerous remains open.