Ever since Google Analytics rolled out sampling I've noticed weird things happening to the data. For example, when looking at All Visits on any report, things look okay with the sampling slider all the way up. But - when you drill down to any one of the predefined or custom advanced segments, things get wacky.
I discovered this when comparing non-bounce visits over the first half of 2012 and 2011. I noticed the line for 2011 had a huge dropoff at one point (View a screenshot). To check what Google was giving me, I looked at the visit number for a particular day under different time ranges: that day alone, the whole month, and the half year.
You can see I'm getting wildly different numbers for Feb. 9 visits depending on the time range selected. It happens anytime I use an advanced segment. It essentially makes using advanced segments in my year over year analysis useless.
Is anyone else seeing this?