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Data changing when I apply a segment

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# 1
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When I apply a segment to my data in Google Analytics and use a wide date range, the number of sessions and pageviews for the "All Users" data and the number of sessions and pageviews for the segment's data is different than the number of sessions and pageviews for the "All Users" data and the number of sessions and pageviews for the segment's data that appear when I use a shorter date range. Why is this happening?

 

I've also noticed that when I apply the segment to my data, the percentage of new sessions for "All Users" that appears when the segment is applied is different than the percentage of new sessions for "All Users" that appears when the segment is not applied (and this even occurs when I use a short date range). Why is this happening?

 

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Data changing when I apply a segment

Top Contributor
# 2
Top Contributor

Hi Kalyd,

 

Are you running in data-sampling issues [1] when viewing a wider date range? You'll see "This report is based on x% of sessions" then and x being less than 100. It often appears with applying segments because that's an ad-hoc report.

 

Best, Christoph

 

[1] https://support.google.com/analytics/answer/2637192?hl=en

Data changing when I apply a segment

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# 3
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Thanks for your response. Yes, I do see "This report is based on x% of sessions" with x less than 100 when I view a wider date range. So that is likely part of the problem.

 

However, I'm still wondering about my second question. Even when I apply the segment to my data and I do NOT see it saying "This report is based on x% of sessions", the percentage of new sessions for "All Users" that appears when the segment is applied is different than the percentage of new sessions for "All Users" that appears when the segment is not applied. Do you know why this is happening?

 

Also, I have a general question. If a segment is created to exclude spam, does that segment ever exclude real users?

Data changing when I apply a segment

Top Contributor
# 4
Top Contributor

How large is that difference you're quoting?

Data changing when I apply a segment

Visitor ✭ ✭ ✭
# 5
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Well, regarding the percentage of new sessions for "All Users" changing when I apply the segment, the difference is not huge - it is about 5 percentage points or less of a difference. But it still concerns me a bit, because I'm not sure which percentage is the correct one - whether it's the percentage that appears when I apply the segment or whether it's the percentage that appears when I don't apply the segment. 

 

Also, you mentioned data-sampling issues with the wider date range in your earlier message. When data sampling occurs, are the metrics provided actually inaccurate?

Data changing when I apply a segment

Top Contributor
# 6
Top Contributor

Okay, Kalyd, I see what you mean.

 

I've tried a custom report with sessions and %new sessions as metrics - there's actually no difference if applying a segment or not! 

 

Maybe it's just the preview numbers in special reports like Audience > Overview that might go (a little) wrong for some reason?

 

Surely sampled data may get inaccurate - see [1] for example.

 

Christoph

 

[1] http://www.lunametrics.com/blog/2016/03/03/accurate-sampling-google-analytics/

Data changing when I apply a segment

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# 7
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Thanks for letting me know. Also, I have a general question. If a segment is created to filter out spam, does that segment ever exclude real users?