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Program behaviour characteristics were examined using data gathered from real program executions. Experiments were performed in a segmented virtual memory with a working set policy; the analyzing costs were kept low using an efficient data reduction method. Empirical results were obtained concerning the influence of the window size on program behaviour characteristics, the accuracy of some average working set size approximations and the sensitivity of program behaviour to the program's input data. These results show that some commonly used assumptions concerning program behaviour are inaccurate. Also there seem to exist “ill-behaving” programs, the behaviour of which does not correspond well with results obtained earlier. The effects of real-time delays during program execution were considered using a new simple method. As an additional experiment, segmenting and paging were compared using various performance statistics; the results seem to favour segmenting.