1. A method for processing data for a system model including the steps of providing a model specification having a plurality of types of items including at least one first item type wherein first associated data is obtained from data input into the system and at least one second item type wherein second associated data is obtained from an operation performed on first data associated with at least one third item stored in a first database, inputting second data into the system, searching the input data for said at least one first item type, storing said data associated with said at least one first item type in the first database, reading the or one of the second item types in a determining step including determining whether the first database includes the or each prerequisite item necessary to determine the one second item type by obtaining third associated data from an operation performed on third data associated with at least one third item stored in the first data base, storing the one second item type in the first database if the or each prerequisite item is present, successively reading each other second item type and storing said each other second item type in the first database if the or each prerequisite item is present in the first database and outputting an indication that the system model can be produced if items of the model specification are stored into the first database.