Friday, December 4, 2015

Counting Steps With Multiple Devices

David Smith:

As you go about your daily life with an iPhone and Apple Watch each is constantly trying to measure your movement to determine if you are walking. Depending on what your current activity is, one or the other will be doing a better job of capturing what you are doing.

If, for example, you are pushing a stroller the iPhone in your pocket will do a much better job than the watch on your wrist. Conversely if your iPhone is in your purse, the watch on your wrist is much more accurate. The challenge this update solved is how to merge these two data sources in a way that provides a consistent and reliable picture of your day.

You might be wondering why I don’t use Apple’s merging system for this. After extensive testing about how that works I determined that it doesn’t really do a good job for step data. The Apple Health algorithm works around the concept of a ‘priority’ device. This priority device’s steps are then used in all instances except where that device is completely unavailable. In which case the secondary devices data is used to fill in the gaps.

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