The Ahlbom pooled analysis is a highly influential analysis of the results from 9 epidemiological studies of magnetic fields and childhood leukaemia. See also a discussion of the study, the detailed results, and the abstract. This page discusses alternative ways of presenting the results.
The paper uses four categories of magnetic field and the normal way of presenting the results is to show the risk ratios in these four categories:
This is suggestive of what has become the commonest interpretation of the results: that there is a raised relative risk in the top exposure category, fields greater than 0.4 μT, but little evidence of increased risk in the lower exposure categories.
This presentation gives the four categories equally spaced. But the categories are not in fact equally spaced in terms of field. The paper itself does not give the average field in each category, but we can make intelligent guesses, and plot the results against the average field in each category:
This presentation is more suggestive of a smoothly increasing risk. In fact, the paper itself performs what is known as a "continuous analysis" and finds a relative risk of 1.15 per 0.2 μT increase in exposure. We can plot this continuous estimate of risk:
On this interpretation, there is a raised risk at low fields. But also, the increase in risk at higher fields is less than suggested by the original presentation. Instead of the risk being doubled for fields above 0.4 μT, the risk does not become doubled until 0.99 μT.
Finally, these presentations are all very dependent on the last point - the risk in the top category >0.4 μT. This is a wide category - the average field in it is 0.7 μT. What happens if we split it up? The Ahlbom paper does contain some further results for risks in categories 0.3-0.4-0.5 and >0.5 μT (and again we have to make educated guesses as to the average field in each category), but unfortunately not with confidence intervals:
This presentation suggests there is more variability in the results and perhaps any attempt to see a pattern is going to have limited success.
