Thank you for highlighting the MJFF initiative. I find it very exciting.

Theoretically you should be able to take (integrate) accelerations to get velocities and you should be able to take velocities to get positions. But what you find in practise is that small errors build up and soon you have meaningless results. This is the "drift" that TexasTom refers to. You can improve the situation by using better algorithms or by collecting sanity checking data from magnetometers or gyroscopes.

I spent a lot of time on this problem before realizing that in some situations, and I think we have one in measuring bradykinesia, it is unnecessary to derive the velocity or position from the accelerations. We can use as our proxy measure of PD the accelerations directly, and to make it simpler still, rather than work with the individual components of the acceleration vector, we can work with the magnitude of the accelerations.

There is great advantage using a smart phone app for data collection. However, it is important to remember that the real aim is therapy. So, as we go forward, I think that whatever approach to measurement is used it should be adaptable enough to form part of intelligent, personalized, dynamic therapies. As Marx wrote:

"The philosophers have only interpreted the world, in various ways. The point, however, is to change it."

John