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Abstract:
This communication deals with the use of meta-analysis as a valuable tool for the synthesis of food safety research, and in quantitative risk assessment modelling. A common methodology for the conduction of meta-analysis (i.e., systematic review and data extraction, parameterisation of effect size, estimation of overall effect size, assessment of heterogeneity, and presentation of results) is explained by reviewing two meta-analyses derived from separate sets of primary studies of Salmonella in pork. Integrating different primary studies, the first meta-analysis elucidated for the first time a relationship between the proportion of Salmonella-carrier slaughter pigs entering the slaughter lines and the resulting proportion of contaminated carcasses at the point of evisceration; finding that the individual studies on their own could not reveal. On the other hand, the second application showed that meta-analysis can be used to estimate the overall effect of a critical process stage (chilling) on the incidence of the pathogen under study. The derivation of a relationship between variables and a probabilistic distribution is illustrations of the valuable quantitative information synthesised by the meta-analytical tools, which can be incorporated in risk assessment modelling. Strengths and weaknesses of meta-analysis within the context of food safety are also discussed.
1. Introduction
Meta-analysis refers to the statistical analysis of a large collection of results from individual studies, such as experimental studies, opinion surveys and causal models, for the purpose of integrating the findings (Glass, 1976). Although interest in synthesizing findings dates back to the work of Yates and Cochran (1938) who combined results of different agricultural studies, it appears however that the sole introduction of a term for this collection of studies (‘meta-analysis’ as coined by Glass,1976) led to an upsurge in the development and application, principally in medicine and social sciences. The primary aim of meta-analysis is to produce a more precise estimate of the effect of a particular intervention or treatment, with an increased statistical power. Since different primary studies are performed using different populations, different designs and a whole-range of other specific factors, it has been suggested that combining them would produce an estimate that has broader generalisability than is possible using only a single study (Sutton et al., 2001). Additionally, meta-analysis can be used to get an insight of the sources of heterogeneity or differences among the results of the primary research. In this sense, meta-analysis not only investigates the reported results of the studies but all aspects of research designs that produced them, such as theoretical constructs, operational definitions of variables, population samples, data collection procedures, statistical analysis, and especially the handling of possible confounding variables that would provide an alternative explanation for the reported results (Noble, 2006).
As stated by Sargeant et al. (2006), the prevention of food-borne illnesses is complex because of the multiple stages in the production and preparation of food. On the other hand, the amount of data produced by food safety research have been growing increasingly in the last ten years, and the advances in information technology are likely to further contribute to this growth. Therefore, there is a need for conducting meta-analysis in the field of food safety, to identify, evaluate and synthesize results, so that policy-makers can access evidence-based and concise information on the effectiveness of interventions to control and prevent food-borne illnesses in humans (Sargeant et al., 2005). Although, in principle, meta-analysis may be conducted to address a broad range of food safety research questions such as effect of interventions pre-harvest (for instance, interventions to reduce faecal shedding of Escherichia coli O157 in beef cattle), effect of interventions post-harvest (for instance, effect of carcass rinsing on Salmonella), disease incidence, prevalence of pathogens, consumer practices, etc., applications in food safety research are still in its infancy. The findings of such independent meta-analyses can offer valuable information on the best interventions and can provide data as input into risk assessment models.
To date, only six published studies using meta-analysis as a tool to combine food safety data have been identified (Patil et al., 2004; Vialette et al., 2005; Sánchez et al., 2007; Bollaerts et al., 2008; Gonzales-Barron et al., 2008; Gonzales-Barron et al., 2009). This article aims to (i) present the general objectives and methodology of meta-analysis and its relevance for the synthesis of food safety research by reviewing two applications, and (ii) to highlight its use in risk assessment modelling discussing its strengths and weaknesses.
2.Connclsions
While meta-analysis is not without limitations (underpowered primary research and publication bias), its systematic approach comprising a broad range of techniques merits consideration among food safety researchers to integrate the current body of knowledge and data on targeted issues along the complex continuum of agro-food production, and to furnish increased credibility to findings in the field. As reviewed in the two metaanalysis applications, while primary studies might be weak or reach contradictory conclusions, the body of information contained in all studies might reveal a clearer picture of the state of knowledge, and can offer valuable quantitative information of effect size in the form of distributions that can be inserted into risk assessment models. Meta-analysis can highlight areas where there is insufficient evidence of the efficacy of interventions, where there is absence of high quality studies, or where there are common methodological flaws in the available research, and can therefore provide direction for future research.
Acknowledgments
The authors wish to acknowledge Safe Food, The Food Safety Promotion Board and the Food Institutional Research Measure (FIRM) administered by the Irish Department of Agriculture, Fisheries and Food.

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