Evaluation of community based injury prevention: an epidemiologist's quandary.

The current and loadable trend in the practice ofmedicine is reliance on sound empirical evidence as aprerequisite for the formulation of treatment proto-cols. Evidence-based medicine, however, is as much arequirement for prevention as it is for treatment for,at least, two reasons: (1) prevention is applicable tothe totality of the population, whereas any particulartreatment concerns a certain sector of this popula-tion; and (2) the cost/eﬀectiveness ratio of the variouspreventive measures depends on a multitude of vari-ables, the values of which have powerful sociode-mographic and cultural determinants (for example,smoke detectors are more cost-eﬀective where woodrather than stones or bricks are used for house con-struction). Paraphrasing from Miettinen’s commen-tary ‘‘The modern scientiﬁc physician’’ [1], it may bemore appropriate in the area of injury prevention touse the term ‘‘scientiﬁcally based public healthinterventions’’ rather than ‘‘evidence-based interven-tions’’, since logic is as powerful a component of thescience of prevention as empirical evidence is. Forinstance, several measures of passive prevention inthe automobile industry rely on laws of physics anddeductive reasoning rather than on formal empiricalresearch in humans.Primary prevention of injuries relies on identiﬁca-tion of causes, whereas secondary prevention of inju-ries presupposes some understanding of thepathophysiologic and mechanistic considerations ofthe induction of injury. Identiﬁcation of causes andpathogenetic mechanisms may be accomplishedthrough various processes of logical argumentation,through experiments in artiﬁcial non-human settingsor through human studies, which can be either ofinformal clinical nature or formal epidemiologicaldesign. Knowledge of causes, however, cannot beeasilyanddirectlytranslatedintopotentialforeﬀectiveprevention.Asinotherﬁeldsofprevention,theoreticaleﬀectiveness should be distinguished from population(community) eﬀectiveness – and it is the latter thatmatters for public health professionals [2, 3].There are several rings in the chain between theo-retical and population eﬀectiveness and the chain canbe broken at any one of them. To take a simpleexample, the eﬀectiveness of seatbelts in preventingserious injuries has been known for more than ﬁvedecades. Nevertheless, their theoretical eﬀectivenesswas compromised either because individuals did notappreciate their potential, because seatbelts were notavailable in all cars or because the states did not haveor were not willing to enforce respective legislation.Whereas logic and laws of physics may be suﬃcientfor estimating theoretical eﬀectiveness only studies inhumans can reliably ascertain population eﬀective-ness of injury prevention measures.Evaluation of population eﬀectiveness can focuseither on intermediate (process) or outcome mea-sures. Thus, a campaign to increase helmet use aimsat reducing head injuries but it can be evaluatedthrough ascertainment of the prevalence of helmetusers before and after the intervention, under the veryreasonable assumption that after the crash a helmetwill reduce the likelihood of a head injury [4].Focusing on process measures greatly increases sta-tistical power because the intermediate state is farmore common than the ﬁnal outcome and the rele-vant latency is also far shorter. Implicit, however, isthe realization that it would be extremely diﬃcult todocument through observational studies an actualreduction of serious head injuries through helmet use.Why has empirical research, including randomizedcontrol trials, been generally unable to documentsubstantial reductions in preventing injuries followingcommunity intervention programs [5]? This is becauseinjuries represent a wide range of etiological andmanifestational entities, each of which is responsiblefor a small fraction of the whole spectrum. Injuryprevention interventions focus on no more than ahandful of practices and only a couple of them can berealistically expected to be eﬀective in any particularsetting. It is an unusual situation when the prevalenceof the targeted exposure is suﬃciently high or the rel-ativerisk for an injury associated with this exposure isconsiderably elevated that the eﬀectiveness of theintervention can be documented with suﬃcient statis-tical power. And the eﬀectiveness of the interventionitself may depend on a number of factors (intensity,duration, repetition, etc), which may or may not beoptimally reached in any particular setting.Focusing on high-risk group interventions, such asprevention of falls among elderly in nursing homesmakes the intervention amenable to outcome evalu-ation because the outcome is not very rare and inju-ries from falls represent the principal injury among