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My organization has recently noticed an issue when applying advanced segments to visit counts during different time ranges.

With no advanced segments turned on, here are the visit counts for Oct 1st - Oct 4th during the time range Sept 8th - Oct 8th:

Oct 1 - 7
Oct 2 - 7
Oct 3 - 8
Oct 4 - 5

Again, with no advanced segments turned on, here are the visit counts for Oct 1st - Oct 4th but I've changed the time range to Oct 1st - Oct 4th. As expected, the numbers are the exact same as above:

Oct 1 - 7
Oct 2 - 7
Oct 3 - 8
Oct 4 - 5

Now, I turn on the "Non paid search traffic" advanced segment. Here are the visit counts for Oct 1st - Oct 4th during the time range Sept 8th - Oct 8th:

Oct 1 - 0
Oct 2 - 0
Oct 3 - 0
Oct 4 - 2

Here is where it gets weird. I keep the advanced segment on, and change the time range to Oct 1st - Oct 4th. This is what I get for the exact same dates as above:

Oct 1 - 4
Oct 2 - 2
Oct 3 - 6
Oct 4 - 5

We've found the same inconsistency in our other GA profiles that get much more traffic (the above numbers come from one of our specialized topic blogs), but the inconsistency is less pronounced where there are more visits. My question is: why are the visit counts different for different time ranges when advanced segments are turned on, but exactly the same when no advanced segments are applied? Is this a GA bug or am I missing something about how the advanced segments work?

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I was unable to reproduce this error. Is this specific to that one advanced segment, or does it occur with any advanced segment? –  Moses Oct 9 '12 at 18:08
    
I tried search traffic and direct traffic and got a similar result. We've also seen it on custom advanced segments. –  user671201 Oct 9 '12 at 18:22
    
I have seen this same thing... things seem to go wonky when you start using advanced segments. I suspect that part of the answer is the algorithm Google uses in selecting sample data, but will be interested in seeing what arises here. –  Dallas Oct 10 '12 at 2:21
    
The sample selection algorithm is my best guess as well, but the numbers are so different! as this forum post points out it's causing enough variation that year over year analysis with advanced segments becomes useless. –  user671201 Oct 10 '12 at 14:11
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