When the computer is involved in a lengthy internal process users do not know if they have the opportunity to carry out another task or take a break.
If the user does not have access to the estimated time of completion of the process, or an indication of how it is progressing, then they may stop the operation just when it is about to end, or they may wait around by the screen unnecessarily when they might as well have gone for a cup of tea, or carried out another task.
This problem has been addressed in some applications by the use of a measuring bar or percentage or text representation to tell the user how much more work there is to do. Examples would include the mechanism on many Web browsers showing that, say, 45% of a site has been downloaded; alternatively, the “copy” operation in the Apple Mac desktop tells the user how many files are left to be copied both in textual and graphical format.
Therefore:
If the timing can be calculated, give an indication of the time remaining, either as a figure, or graphically (as a time bar, as a clock, etc.).
If timing can not be estimated, but the process has identifiable phases, give an indication of the phases completed, and of the phases remaining.
If neither of these possibilities exist, then at least indicate the number of units processed (e.g., records, vectors, etc.).
. . . .