We present the design of a test for measuring students' abilities concerning the application of control structures. Validated test instruments are a valuable tool for the evaluation of teaching both in a research setting as well as in a classroom setting. Our test is based on item-response-theory, in particular the Rasch model, and comprises a set of items all following the same format and using a simple, artificial programming language. We field-tested and modified the instrument in four iterations using only small samples and special statistical methods instead of the large samples usually required for IRT models. After the fourth iteration, the test has now reached a usable state. Based on the results, we were able to identify two misconceptions that are occurring very frequently in our test population - students of grade 7 to 10 in secondary schools.