The Statistics Cache manages the data used in computing student performance statistics.
Much of the time spent computing statistics is actually used to compile student data into a more useful format; the temporary course cache. Normally student data is maintained in an archived format, which does not provide for rapid access. Since each student's data is stored on their home server in their account, and not with the course, it takes time to compile the data and store it in the temporary course cache. All of the statistics modes use the same cache so the process of compiling the student data does not need to be repeated when successive statistics are requested.
Only those students who are involved in the current calculation will have their data compiled. When you modify the selected set of students, by changing the section you are computing statistics with, the caches will be updated.
The temporary course cache is maintained for 2 days on the server used for the statistics computation.
The cache management buttons, marked Clear Caches and Update Caches, appear when the temporary course caches are in use.
Clear Caches causes the entire cache for the course to be flushed, discarding all the current contents. The cache is then rebuilt to display the desired statistics. The normal use of statistics should not require you to press this button, but it is here for completeness.
Update Caches checks the students' records for updates since when their data was compiled. Any changes to the students' record for your course will be reflected after this button has been pressed. The Update Caches button is useful if your students are currently completing a homework set, and you wish to evaluate their progress in the timeframe your statistics were generated.