BMJ  2003;327:E240 (4 October), doi:10.1136/bmjusa.03070004 (published 30 July 2003)

Following are edited excerpts from Rapid Responses generated by this article, which can be read in their entirety at http://bmj.com/cgi/eletters/326/7398/1083. — Editor

BMJ USA: Letter

RAPID RESPONSES FROM BMJ.COM

From BMJ USA 2003;July:395

Editor — I have often have heard primary care physicians and nurses claim that "in their experience" patients' cholesterol levels fall significantly after they have received dietary advice. This is unlikely. With the benefit of a control group, it is apparent that even intensive dietary advice has a small effect on cholesterol levels. The mistake arises because we fail to appreciate just how variable a single cholesterol measurement actually can be. We also tend to start from a premise of belief in our beneficence, rather than scientific skepticism. Questioning professionals' views of their own efficacy is a fast track to unpopularity and poses a significant obstacle to understanding variation.

Brimingham niversity, Birmingham, UK. T.P.Marshall{at}bham.ac.uk

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Editor — We disagree with the following statement in the section on public health: "The policy of vaccinating children against meningitis was introduced at a time of heightened incidence. The headline benefit of a 75%-90% reduction in cases is an overestimate as most of the reduction would have been due to the regression to the mean."

When assessing interventions applied at an individual level, such as vaccination, it is possible to measure the direct effect of the intervention by comparing incidence rates in those who did and did not receive the intervention. For the meningococcal group C vaccine the 75%-90% reductions reported were based on this direct effect and not on a simple comparison of incidence before and after the vaccination campaign.

Even if the comparison were between pre- and post-vaccination incidence it would still not make sense to conclude that most of the effect is due to regression to the mean. The incidence of group C disease had risen steadily for a number of years prior to the introduction of the vaccine. Large random annual fluctuations did not occur, so there was no good reason to expect that the introduction of vaccination would be followed by regression to some lower underlying mean level. This would not explain even a trivial proportion of the observed reduction.

To determine whether regression to the mean provides an explanation for all or part of an observed change, it is necessary to understand the contribution of both the random and systematic components of variation. The temporal patterns for many diseases (as well as other events) exhibit cyclical trends, which include regular epidemics, and these predictable changes need to be considered when evaluating interventions. The random components that are the basis for regression to the mean are often relatively small.

Communicable Disease Surveillance Centre, London, UK. nick.andrews{at}hpa.org.uk

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