  In this paper, we first define a difference measure between the old and new
sequential patterns of stream data, which is proved to be a distance. Then we
propose an experimental method, called TPD (Tradeoff between Performance and
Difference), to decide when to update the sequential patterns of stream data by
making a tradeoff between the performance of increasingly updating algorithms
and the difference of sequential patterns. The experiments for the incremental
updating algorithm IUS on two data sets show that generally, as the size of
incremental windows grows, the values of the speedup and the values of the
difference will decrease and increase respectively. It is also shown
experimentally that the incremental ratio determined by the TPD method does not
monotonically increase or decrease but changes in a range between 20 and 30
percentage for the IUS algorithm.
