Rob van Hest Capture-recapture Methods in Surveillance - RePub ...

Chapter 11
Question 3:
What is the feasibility and validity of truncated population estimation models in
infectious disease surveillance?
For priority setting, service planning and resource allocation it is necessary to know the
number of persons in a targeted group. This number can also be used to assess the
coverage of an intervention. Often direct (enumeration) techniques are not feasible to
estimate the size of hidden populations. Instead, indirect techniques such as capturerecapture
analysis have to be used. Paradoxically, for hidden populations often the
preferred three linked registers, allowing for log-linear capture-recapture analysis in order
to reduce bias in the estimates, are not available. As an alternative, truncated models,
related to capture-recapture analysis, and applicable to frequency counts of observations
of individuals in a single source of information, are described in the literature. Two such
truncated models are Zelterman’s Poisson mixture model and Chao’s heterogeneity
model. 24-26 These models aim to estimate the number of unobserved persons in the
(truncated) zero-frequency class based upon information from the lower observed
frequency classes, assuming a specific truncated distribution of the observed data, e.g.
Poisson in Chao’s model. Observed frequency distributions may not be strictly Poisson
and to relax this assumption Zelterman based his model on a Poisson mixture
distribution, allegedly allowing greater flexibility and applicability on real life data. The
validity of truncated model estimates depends on the possible violation of the underlying
assumptions, similar to capture-recapture analysis as described earlier, although the
independent registers assumption is replaced by the constant (re)observation probability
assumption when using a single data source. In addition, equiprobability (i.e. equal
ascertainment probabilities of all registers) should be assumed when using multiple
sources. This violation could be as much, possibly even more, as for capture-recapture
analysis. 11
We estimated the coverage of a tuberculosis control intervention, a targeted
mobile tuberculosis screening programme among illicit drug users and homeless persons
in Rotterdam. Application of truncated models was feasible because this screening
programme uses a single register. Although capture-recapture techniques for estimating
the size of a population from a single register have been described occasionally, 27-29 for
feasibility one prefers the simplest technique with almost similar assumptions. We could
extract, check and prepare the required data from the existing routine dataset in two days
and calculate the point estimates on a pocket calculator. Violation of the perfect recordlinkage
assumption is considered minimal because of good computerised and visual
identification of the clients in the screening programme. However, the closed population
assumption is violated because every year a substantial number of people not previously
screened enter the programme, resulting in under-estimation of the coverage. We cannot
exclude heterogeneity among individuals belonging to the target group of the screening
programme but this could cause limited bias in the model estimates. Truncated models
are arguably more robust to violation of the homogeneity assumption since they are partly
based upon the lower frequency classes, assumed to have more resemblance to the zero
frequency class. The constant (re)observation probability assumption will be violated as
well to some extent but this effect could be limited due to the nature and organisation of
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