# Sum of totals of Daily Unique Visitors of sites not equal to total of Daily Unique Visitors of site collection

I am replacing a colleague for the support of a SharePoint 2010 intranet.
The root site of a site collection has two subsites.
I looked at the Web Analytics summaries of the site collection, its root site and the two subsites.
The summaries showed data for the preceding 30 days.
I saw the following total numbers of Daily Unique Visitors on the summaries :

• On the summary of the site collection : 18,132
• On the summary of the root site : 17,972
• On the summary of the first subsite : 12
• On the summary of the second subsite : 3,189

The sum of the total numbers of Daily Unique Visitors of the sites is 21,173 (= 17,972 + 12 + 3,189) which is not equal to the total number of Daily Unique Visitors of the site collection (18,132).
Can someone explain me why ?

Any help will be greatly appreciated

Think this one through logically in an imaginary scenario: It is now 12:00 AM so we reset the counters to zero -

• A) Summary of Site Collection: 0
• B) Summary of Root Site: 0
• C) Summary of First Subsite: 0
• D) Summary of Second Subsite: 0

Next we load the root site into our browser then look at the stats:

• A) Summary of Site Collection: 1
• B) Summary of Root Site: 1
• C) Summary of First Subsite: 0
• D) Summary of Second Subsite: 0

You are assuming A=B+C+D, so everything looks good. But now, if I go to Second Subsite, here's what I would see:

• A) Summary of Site Collection: 1
• B) Summary of Root Site: 1
• C) Summary of First Subsite: 0
• D) Summary of Second Subsite: 1

Since we are talking about unique users, even if I started by going to the Root Site, it will not increment because the same user has already been there; it will increment on Second Subsite because I haven't been there today.

Clearly, our formula A=B+C+D does not work. I would suggest that it is more appropriately expressed "A is equal to the count of unique values contained in the SET(B,C,D)" instead of the sum of each.