AbstractA method is described for assigning beef cattle performance records to within-herd management effects (contemporary groups) retrospectively, using information on the recording date and herd. This method takes account of the within-herd calving pattern. The objective of developing a method of assigning records to contemporary groups was to maximize the contemporary group size while ensuring as far as possible that records in any contemporary group were subject to the same management.A simple method for assessing the effectiveness of different strategies for assigning records to contemporary groups is proposed, based on investigating the balance between the effects of contemporary group size upon the heritability and the accuracy of mass selection.The results of a study of different contemporary grouping strategies in the Simmental breed are reported.