1. A software method for assigning a value to at least one data item of a test ease instance, comprising a plurality of machine-readable instructions stored on non-transitory computer readable storage medium for instructing a tester computer to carry out the steps of: selecting an input data item from a test design library, the library further comprising, a plurality of input data items each defining a predetermined test design, information about semantics and data type of the input data items associated with each of said plurality of input data items; each of said plurality of input data items including an index associating that input data item to said information about semantics and data type of that input data item, the index containing a key value, and wherein the indices of two or more input data items may share the same key value; assigning the selected input data item as a value of data item of the test case instance; and executing said test case instance with assigned data items by sending corresponding test messages from said tester computer to a system under test and receiving response messages at said tester computer from said system under test.