Maternal age and risk of type 1 diabetes in children

doi:10.1136/bmj.322.7300.1489/a

BMJ 2001;322:1489 ( 16 June )

Letters

Maternal age and risk of type 1 diabetes in children

Flawed analysis invalidates conclusions

Relative risks by maternal age are biased

Other secular trends may explain associations with diabetes risk

Association may disappear after adjusting for year of birth

Authors' reply

Flawed analysis invalidates conclusions

The first 150 words of the full text of this article appear below.

EDITOR $---$ The conclusion drawn by Bingley et al, of dependence on both birth order and maternal age, is a tempting explanationof previous ambiguous results.¹ I believe, however, that theirstudy is seriously flawed in the analysis of data, to an extentthat the results are completelymeaningless.

There are three apparentproblems.

Firstly, the cohort studied consists of children with a sibling diagnosed with type 1 diabetes. Hence every only child inthe study must be a case. Roughly half of the children from twochild families will be cases, and so on. This immediately producesan (inverse) association between the number of siblings and thediagnosis of type 1 diabetes. To see this clearly, suppose thatcases occur purely at random. To make the numbers easy, supposethe incidence is 1%. Now select 1000 only child cases and 1000cases with a single sibling. Almost half of the 2010 expectedcases . . . [Full text of this article]

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