A chain risk model (CRM) has been developed to calculate the cost-effectiveness of phytosanitary measures. The CRM is a quantitative model for phytosanitary assessments of all kinds of agricultural product chains. In the CRM, chain stages are connected by the product volume streams at which infections of pests can be spread from one stage to another. The arrangement of the stages in the chain can be varied by the model's user. Primary infection of a pest in the chain stages can take place by imported commodities, from a former chain stage and from outside. Spread of pests is simulated by a spread factor and the multiplying factor of the product itself, estimated by the model user. The costs of measurements assessed and their effect on the pest or detection of the pest have to be entered in the model. The result of the model calculations is the cost effectiveness of a measurement or set of measurements. An additional result is an overview of the distribution of infected lots in the chain. Experience with the CRM has been gained by applying it to two actual problems: Clavibacter michicanensis subsp. michicanensis, which causes bacterial wilt and canker of tomato (Lycopersicon esculentum L.) and Potato spindle tuber viroid (PSTVd) in potato (Solanum tuberosum L.), which is also called tomato bunchy top virus. Both cases confirm the hypothesis that CRM is applicable for the assessment of various sets of measurements, like checks in different chain stages and application of a quality care system in some stages. The CRM is primarily intended for use by the Dutch Plant Protection Service (nVWA) but can also be used by other national plant protection organizations. The generic and flexible model can be used for calculations on topics as assessment of new detection techniques, assessment of risk reduction measurements on pathways and assessment of different alternatives of sampling in relation to different kinds of phytosanitary checks.
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