Completeness of birth address data
Editor – In the paper by Draper and colleagues , the authors state
in their conclusions that “there is no obvious source of bias in the
choice of cases or controls”. Although we agree with the authors in their
statement that “registration for childhood cancer is nearly complete”, we
are concerned that the cases included in the analysis group may be prone
to censoring since the inclusion criteria was based on cases both born and
diagnosed in England and Wales between 1962-1995.
Cases extracted from the National Register of Childhood Tumours
(NRCT)  were diagnosed between 1962 and 1995 aged under fifteen years
and the analysis was restricted to those born during the same period. The
aim of the study was to investigate whether there was a relationship
between distance of the home address at birth and high voltage power lines
for the subsequent development of childhood cancer. This approach will
have excluded a proportion of individuals diagnosed with cancer under the
age of 15 years but who were born after 1980 and diagnosed beyond 1995. A
complete birth cohort from 1962-1995 would require ascertainment of cases
In addition, there were a number of missing birth addresses (n=1700,
5%) and we would be interested to know whether the proportion differed
across the study time period. Our experience would suggest a higher
proportion of cases with missing birth address records near the beginning
of the study period. Birth address postcodes were not routinely available
during the 1960s possibly reducing the completeness of the cohort during
the early period.
The main aim of the study was to examine risk around the time of
birth and therefore we believe that a more appropriate analysis would have
been one where the inclusion criteria are based on a complete birth
cohort. It would be interesting for the authors to re-analyse their data
by restricting cases to those diagnosed from 1962 onwards and born between
1962 and 1980 to minimise any potential effect from censoring.
1. Draper G, Vincent T, Kroll ME, Swanson J. Childhood cancer in
relation to distance from high voltage power lines in England and Wales: a
case-control study. BMJ 2005;330:1290-2.
2. Stiller CA, Allen MB, Bayne AM, et al. United Kingdom: National
Registry of Childhood Tumours, England and Wales, 1981-1990. In
International Incidence of Childhood Cancer: Volume 2. eds Parkin DM,
Kramárová E, Draper GJ et al. pp.365-367. Lyon: IARC Scientific
Publications No 144, 1998.
Competing interests: No competing interests