This reminds me of when our "business intelligence people" moved from WebTrends (I think) to Google Analytics. I remember a manager saying: If I hadn't worked with webtrend's data for a couple of years, I'd have no idea what the Google Analytics data means. The reason being, that how Google Analytics counts sessions/visits/bots etc is completely opaque.

So the trends were similar, but the actual numbers where completely different.

I don't understand why people will use "analytic tools" that doesn't actually document what they're counting, and how. At least with Free software, one can have a look and try to figure it out (eg: piwik, visitors). With a lot of "services" your only option for a sanity check is to look at access logs -- which of course is one of the things one tries to get away from when moving to an analytics platform...

(Note: Haven't yet looked at what documention amplitude provides, just noting that documenting how you're counting is actually an essential part of the product -- and something any user needs to know. Seeing your comment here, at leasts hints that that information isn't (readily) available on amplitude.com/docs. If it is, you'd just have linked to it, right? ;-)