To change the number of points in the derivative calculation, Choose Settings or Settings For FILENAME from the file menu. Then adjust the number of points for derivative calculations and select OK.
Logger Pro and LabQuest use a derivative function for some tasks, including calculating velocity and acceleration from Motion Detector data. See TIL 1011: How does Logger Pro calculate velocity and acceleration from Motion Detector data?
There are a few cases when one might wish to adjust the number of points used in a derivative calculation. Using a greater number of points results in a smoother derivative graph, but the effect of changes in the source data is smeared in time. On the other hand, using fewer points for the derivative calculation results in a more responsive graph with more noise.
For example, in an experiment where a person is walking in front of the motion detector, using 15 points for derivatives will result in quieter, more easily interpreted velocity graphs. The temporal averaging will not affect interpretation.
On the other hand, in a mass-on-a-spring simple harmonic motion experiment, using more than 5 or so points will often result in excessive averaging, altering the magnitude of the peak velocity and acceleration values.
It is easy to observe the effect of changing the number of points used in derivatives, as the setting takes effect immediately, acting on existing data. One can optimize the setting as needed.
Another case in which you might want to reduce the number of points in a derivative calculation is when you have fewer data points in the source column.
The default number of points for the derivative calculation is 7. The setting rides with the file, meaning that if you alter the setting and save the experiment file, the setting will be restored when the file is later opened.