Epidemioloog Kabat reageert op Helena onderzoek
Het meer dan één jaar oude Helena onderzoek dat deze week plotseling in het British Medical Journal verscheen roept sterke weerstand op. In reacties (Rapid Responses) op het artikel wordt de vloer aangeveegd met het onderzoek waarover wij hier al eerder berichtten.
Gisteren voegde zich ook Epidemioloog Dr. Kabat, die vorig jaar de resultaten van een eigen, massaal onderzoek over de materie publiceerde, bij de kritiek:
The attempt to make claims about the effects of smoking bans based on this very weak ecologic study raises disturbing questions about our ability to distinguish between sound science and wishful thinking.
The authors reported that the number of heart attacks within the city of Helena dropped by 40% immediately following the initiation of the ban. They claim that this is powerful evidence for an effect of exposure to secondhand smoke on heart attacks. But if we look at their data we see just how questionable this claim is.
First, the researchers only had information from hospital records on where a person lived. They did not interview the patients, so they had no information on whether their exposure to secondhand smoke changed as a result of the ban. They also did not present any information on whether smoking habits were affected by the ban. The fact that they had no information on exposure is a major deficiency.
Second, the drop in heart attacks is based on very few cases: 4 per month on average during the ban compared to 7 per month before. Due to these small numbers the reported difference could easily be due to chance or to some uncontrolled factor. The number of heart attacks in the area outside Helena was even smaller. It should not be surprising that, given these small numbers, there are fluctuations of the magnitude seen in this study.
Finally, the “immediate effect” and its magnitude really should make anyone stop and question the connection the authors are asserting. There are few interventions in public health that have such an immediate effect. Even if all active smokers in Helena had quit smoking for at least a year, one would not expect to see such a dramatic effect. No previous epidemiologic study or community smoking cessation program has ever shown that a reduction in smoking or exposure to secondhand smoke causes an immediate decline in heart disease incidence or mortality.
A rigorous study would have involved collecting information on a population of adequate size and interviewing individuals to assess their pre-existing risk for heart disease as well as how their behavior was affected by the ban in order to make a plausible connection between the two phenomena.