The invention relates to a method (1) for determining supporting points of a test plan (9) for measuring pre-defined test variables of a test machine on the basis of previously measured operating values (3) of operating variables of at least one field machine during the normal use thereof. In an aggregation step (2), the detected operating values (3) are allocated to categories (4) with regard to at least one selected operating variable, according to a predefined classification rule. Default variables are selected in a default step (5) before or after the aggregation step (2). The default variables form at least one subset of the operating variables. The operating category frequency (7) for each category (4) is determined in a determination step (6) following the aggregation step (2). In a subsequent determination step (8), the supporting points of the test plan (9) are determined on the basis of the operating category frequency (7). The supporting points are determined in the determination step (8) such that a deviation of a relative test category frequency, on the basis of the test plan (9), of determined default values of the default variables, associated with categories (4) according to the classification rules, from a relative operating category frequency of the operating values (3) classed according to the classification rules, of the operating variables corresponding to the default variables, is minimised according to a predefined optimisation criterion.