Colin Wyers takes some time to dig under the surface of ball-in-play data and finds that it's far from a clean set of numbers.

Here's one of the key findings:

Letís face itóno matter how much we massage the data, there simply is not a way to objectively define the difference between a fly ball and a line drive. It is inherently a subjective and somewhat arbitrary distinction. ... a simple stopwatch could provide more accurate, quantifiable data than what weíre getting right now.
If you had a half dozen sources collecting BIP data, it looks like you could get a half dozen significantly different set of findings based on that data. That's a fairly huge problem for the newer defensive metrics.