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Before beginning a risk assessment, the purpose and scope are established (i.e. problem formulation), but it may be useful to revisit these at the initiation of the hazard characterization step. The knowledge gained in previous work may, for example, indicate the need to refine the initial scope. That refinement often requires interaction with the risk managers to ensure that changes in the scope do not affect the utility of the final results.

The initiation of a hazard characterization requires a systematic planning stage to identify the context, purpose, scope and focus of the study to be carried out. Risk assessors should consider aspects of the pathogen, host, and food-water matrix (Figure 3).

Figure 3. The epidemiology triangle (modified from Coleman and Marks, 1998)

Addressing the following list of questions may help structure or refine the problem under consideration:

In addition to the above parameters, it may be appropriate to include consideration of certain possible prevention or protection strategies, or both, such as immunization of a population against hepatitis A or typhoid fever.

These structuring questions need to be considered prior to the onset of hazard characterization. They are best formulated following a communication process between the assessors and the users of the hazard characterization results (risk managers). These questions will orient the collection, collation and evaluation of available information and data. They will also serve to identify data gaps and areas of uncertainty, and provide the risk manager with realistic expectations of the product of the hazard characterization. The answers provided should enhance knowledge of the pathogen and the disease, and identify areas where more research is needed.

A preliminary investigation phase may be necessary to define a risk model. In this phase, qualitative or quantitative evidence, or both, is structured to the overall framework of risk assessment and is used to "map" the risk model. Such studies may assess if sufficient data are available to answer the risk assessment question(s), and will result in a recommendation as to whether the assessment should be qualitative, quantitative, an analysis of the gaps in the data, or whether a different question might be answered with the available data.

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