Evaluating system reliability requires modeling the interaction of resources, information, and material within the system. Such a model must consider quantitative data describing the reliability of each element of the system, as well as logical data describing the relationship between individual components. For example, a manufacturing system may assemble products X, Y, and Z, on machines M1, M2, and M3, respectively, and package the products on a fourth machine, M4. Therefore, three different relationships exist between the components of the system, one for each product. If the reliability of each machine differs, the system reliability will differ depending on the product. Similarly, the reliability of the system as a whole will be affected by the production levels of products X, Y, and Z.