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| Is sleeping in a car seat dangerous? |
Earlier this year, the
Fisher-Price company relented to pressure from the AAP (American Academy of Pediatrics) and recalled 4.7 million of Rock 'n Play (RnP) baby rockers, which now presumably occupy landfills. This recommendation stemmed from an "investigation" by consumer reports showing that since 2011, 32 babies died while sleeping in the RnP. These deaths are tragic, but what does it mean? In order to make sense of this "statistic" we need to determine a rate, based on the exposure period, something like "the rate of infant death in the RnP is 1 per 10 million RnP occupied hours" or something like that. Then we would compare it to the rate of infant death sleeping in bed. If it was higher, we would have a starting point for considering whether
ceteris paribus, maybe it's the RnP that is
causing the infant deaths. We would want to know the ratio of observed deaths in the RnP to expected deaths sleeping in some other arrangement
for the same amount of time. Of course, even if we found the observed rate was higher than the expected rate, other possibilities exist, i.e., it's an association, a marker for some other factor, rather than a cause of the deaths. A more sophisticated study would, through a variety of methods, try to control for those other factors, say, socioeconomic status, infant birth weight, and so on. The striking thing to me and other evidence minded people was that this recall did not even use the observed versus expected rate, or any rate at all! Just a numerator! We could do some back of the envelope calculations with some assumptions about rate ratios, but I won't bother here. Suffice it to say that we had an infant son at that time and we kept using the RnP until he outgrew it and then we gave it away.
Last week, the AAP was at it again, playing loose with the data but tight with recommendations based upon them. This time, it's car seats. In an article in the
August, 2019 edition of the journal Pediatrics, Liaw et al present data showing that, in a cohort of 11,779 infant deaths, 3% occurred in "sitting devices", and in 63% of this 3%, the sitting device was a car safety seat (CSS). In the deaths in CSSs, 51.6% occurred in the child's home rather than in a car. What was the rate of infant death per hour in the CSS? We don't know. What is the expected rate of death for the same amount of time sleeping, you know, in the recommended arrangement? We don't know! We're at it again - we have a numerator without a denominator, so no rate and no rate to compare it to. It could be that 3% of the infant deaths occurred in car seats because infants are sleeping in car seats 3% of the time!
