google analytics
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?
I think it should be possible to create such a report using the Google Analytics Reporting API in combination with the UrlFetch features of Google Apps Scripts. With some programming you could create a Google Docs spreadsheet with normalized data.
You can create a custom inteligence alert.
Go to Home > Inteligence Events > Overview.
From there click on the tab custom alerts and then click on Manage Custom Alerts.
eg:
Best Answer
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.