# Why is Google Analytics data off when selecting multiple days?

When in the Audience > Overview and selecting data for one specific day (4/20), I get a Unique Visitor count of 1,392. When I select the data of another specific day (4/21) I get 942. But when I get the data of the two days together (4/20 - 4/21) I get a Unique Visitor count of 2,252. The math does not seem correct as 1392+942 = 2334 != 2252.

What could be causing this?

## migrated from superuser.comApr 25 '12 at 21:44

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## 2 Answers

Google defines unique visitors as "number of unduplicated (counted only once) visitors to your website over the course of a specified time period" http://support.google.com/googleanalytics/bin/answer.py?hl=en&answer=33087 so what is happening here is when you are looking at each day those numbers are correct for that day. When you expand the date range the numbers are different because you have 82 visitors (2334-2252=82) that came to the site on both days so they are counted as unique when you are looking at a single day but not when you are looking at two days.

• Yep, analytics deals in unique users from an IP basis, to my understanding. – Jason R. Mick Apr 25 '12 at 21:03

Considering 1,000 is for a single day, there is 1,440 minutes in a day... This means approximately 1 per minute.

I am guessing that some people are on the site at just before midnight one day and it only counts for one day...

some sort of rounding error/multiple day thing like this is the only thing I can think of!