Note that the overall ranking of the dominant write command did not change even though the method did. File Write Method Invariant Components Work on the z-curves is still in progress. However it is clear that the z-curve holds well for an entire population over different snapshots of activity. It seems likely that there is predictable variation in the slope for individual users. Most seem to gradually specialise their command use and then back away towards a slightly more general distribution.
High slope corresponds to a more hierarchical use of commands than lower slope.
It may well turn out that the frequency distribution is a function of the interface, task and user. Much work could be done in this area. Figures 4 and 5 show considerable variation in command ranking over time. It is suspected that these changes have to satisfy the overall constraint that the frequencies fit on the z-curve. Where there is no direct substitution available, several ranks may be adjusted.