Preventing Childhood Obesity - Evidence Policy and Practice.pdf

Evaluation of community-based obesity interventions
tion as a black box, with no information on how it
worked, how it could be improved, or what the crucial
components of the intervention were. By including
a process evaluation within the trial, data can be collected
to address these questions, both to inform
future development of the intervention, and also to
contribute to theory and understanding of the relation
between context, mechanism and outcome. Thus, it is
perfectly feasible to allow variation in intervention
form and composition, provided that the basic function
and process of the intervention is standardized. 24
This reproduces realistically within the trial the kind
of variation that will naturally occur in real world
practice, and also allows assessment of key delivery
and context variables that may influence intervention
effectiveness and acceptability.
An example of such an effectiveness trial is a cluster
randomized trial of fruit tuck shops in schools, in
which intervention schools were allowed to implement
the tuck shops in a variety of ways. The process
evaluation assessed the alternative methods and led to
the production of a booklet published by the Food
Standards Agency providing guidance on alternative
models. 25 The trial produced an unbiased estimate of
the overall impact of fruit tuck shops on fruit consumption
and norms, and also identified an important
synergistic interaction between fruit tuck shops
and school policy on foods that pupils were allowed
to bring to schools. 26
Control g roup a llocation and t reatment
If a summative evaluation of a community intervention
is conducted without a control group, then it is
impossible to attribute any measured change in outcomes
to the intervention as this may in whole or in
part be a change that will have occurred anyway. The
measurement of outcomes in a comparison group is
therefore important, although great care must be
taken to minimize selection bias, whereby the two
groups are different in observed or unobserved characteristics.
A common source of selection bias is to
implement an intervention in selected communities
where key stakeholders and partnership organizations
are committed to the intervention, and then to recruit
randomly selected communities to act as controls.
Even if these control communities are well matched
in terms of socio - economic and other characteristics
that may be expected to be related to obesity, it is
always very possible that greater change would have
occurred in the intervention community in the
absence of the intervention, owing to the commitment
and interest of the key organizations and
stakeholders.
Similarly, communities with high levels of obesity,
compared to other comparison communities, will be
more likely to experience favorable outcomes owing
to regression to the mean, rather than any true intervention
effect. Careful selection of intervention and
control communities, and analyses that adjust for
baseline differences, can mitigate the potential effects
of selection bias. However, to minimize selection bias
it is always preferable to allocate communities to
intervention or control conditions through random
allocation, in which each community has an equal
chance of being allocated to the two groups. Such a
design is known as a cluster randomized trial, in which
randomization is undertaken at the level of the community
(or other cluster such as school or workplace),
rather than of individuals. Whether random or non -
random allocation is used, analyses of community
level interventions and sample size calculations must
use appropriate statistical methods that take account
of the non - independence of individuals within
communities. 28
In a controlled experimental or quasi - experimental
research design, the counterfactual, or control group
treatment, must be carefully determined. In an effectiveness
trial of a novel community intervention, it is
not necessary to require the control group to follow a
highly standardized course of relative inaction. It
would be ethically unacceptable to require communities
to withdraw existing policies or actions. A common
solution is to allow all communities within the evaluation
to continue with existing policies and actions
(normal care), with the intervention communities
getting the extra experimental intervention or program
in addition to current activities. Part of the process
evaluation should be to monitor non - intervention
activity in all communities, to provide data on variations
in “ normal care ” across the two experimental
groups.
Outcomes and s ustainability
It would appear to be axiomatic that evaluations of
obesity prevention interventions should include
obesity or body mass index as a primary outcome.
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