PhD thesis - University of Hertfordshire Research Archive

have been increased, necessitating additional statistical power to identify true
differences which might have existed. However, data were captured on
almost 20,000 cases of Campylobacter infection and therefore statistical
power will have been adequate.
The choice of denominator data in disease determinant analysis is important
in ensuring that incidence estimates are not skewed. This is best illustrated in
paper 3 (Campylobacter Sentinel Surveillance Scheme Collaborators,
2003b), where the risk of campylobacteriosis associated with travel to
Cumbria was inflated potentially by increased access to this area of the
country by a number of participating health authorities situated in north or
north west England. This was unavoidable, as health authority-specific
denominator data was unavailable. It is important to bring such caveats to the
attention of the reader so as to keep the findings in context.
In paper 5 (Campylobacter Sentinel Surveillance Scheme Collaborators,
2003a) and paper 8 (Gillespie et al., 2008) we were unable to control for all
the factors under investigation in a single analysis, increasing the likelihood
of uncontrolled confounding. This was especially apparent in the latter, where
age, gender, ethnicity and socio-economic status were all investigated. This
potential drawback could have been overcome by applying, for example,
multivariate log-linked Poisson regression techniques to the data, which
would also have allowed for the highly seasonal pattern of infection to be
examined. Such techniques require denominator data stratified by all factors
under investigation, however, and these were unavailable.
Bearing in mind its advantages and disadvantages, case-case methodology
is applicable to any data where an outcome is readily identifiable and
exposure data are available, from whence they follow case-control study
methodology. That is not to say however, that the analytical methods
employed in the publications described were not improved over the course of
the study. The changing analytical strategy with regard to the case-case
comparisons, described previously, is a case in point. However, there is no
suggestion that the findings of the earlier studies are necessarily
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