Lies, damned lies, and radiation statistics

(A) 1.4, 1.3, 1.1, 2.3, 2.1, 1.6, 0.9; and
(B) 0.4, 1.3, 0.7, 2.0, 1.1, 1.5, 0.9.

In (A), most of the numbers are above 1; when we do the meta-analysis, the result will be above 1 and we can be fairly certain there is  something going on near the reactors. Of course the impact may not be due to radiation from a reactor. Maybe some reactors were built on land once used by coal fired power stations. The cheap land draws young families. The coal stations emitted fly ash having a radioactivity exceeding anything found around nuclear reactors and the kids play on contaminated ground. Correlation isn't causation, so while the numbers would provide a reason for further research, they are not conclusive.

In (B), the numbers are all over the shop. Being near a reactor doubles your risk in one case and more than halves it in another study. When the results are combined - and it's a little more complex than just adding and averaging ... don't worry, the statisticians know how to do it - the final figure may perhaps be above one, it may even be statistically significant, but the fact that there is no clear trend in the data should make you cautious about resting any grand claims on it.

Now here (see below) is an actual set of numbers from the meta-analysis used by Caldicott (with apologies for the poor quality of the PDF to jpg output). It is shown as a forest plot, with blobs instead of numbers. The horizontal lines through the blobs show the 95 per cent confidence interval. If the left end of that horizontal bar is to the right of the 1 line - the line of "no effect" - then the result is statistically significant. Three studies show a reduction in leukemia around plants and while there is a net effect, this would probably disappear without the Hague study. Taken as a whole, the picture is rather less convincing than the net effect cited by Caldicott would indicate.

But there is another way of looking for an impact, if any, of nuclear plants on childhood leukemia.

We could compare the national rates of childhood leukemia in countries with and without nuclear power. If nuclear plants had a big impact and nothing much else caused these cancers, we might expect a discernable difference.

The WHO Globocan database has data on leukemia classified by age and sex in countries around the world. France produces nearly 80 per cent of its electricity with nuclear power stations and the incidence rates of leukemia in 0 to 14-year-olds in the six reporting regions range from 0.9 cases per 100,000 per annum in Bas-Rhin for males through to 5.6 in Isere for females.

In Australia, the rates range from 2.3 for male children in Tasmania, through to 6.8 for females in the ACT. The data in these tables is 1995, but most of the French nuclear plants were operational well before this date and I couldn't find more recent data. These numbers tell us that any effect from nuclear plants is small or zero relative to whatever else is causing these cancers.

Various studies have reported a clustering of childhood leukemia cases, but attempts to pin down the cause, or causes, have been pretty unconvincing. For example, the study linked to in the preceding sentence thought the most likely cause was petroleum processing and distribution network. This analysis suffered some pretty heavy criticism. Later work has found slight clustering in some time periods but not others and drew attention to the difficulties of defining a cluster over time. For example if three children in the same street get leukemia, is it a cluster if the cases are separated by 20 years? The ebb and flow of suburbs and populations makes clustering rather more complex to nail down than appears at first sight.

Most of us have no objections to dental X-rays, CT-scans and a host of other uses of radiation in its various forms. We trust that the scientific understanding of radiation is good enough to rest assured that the little aprons the dentists give us actually work. I could have relied on that same trust to argue that the scientific understanding of radioactivity shows that it is simply impossible for nuclear plants under normal operation to have any impact on childhood leukemia rates. But I've tried in the above article to show that even without such faith, the numbers simply don't add up and don't give any cause for concern.