Everything has to do with the period of time you are looking at, to explain the phenomenon better I have taken a real example from the data of one of my clients. On this example you can see that the number of “Absolute Unique Visitors” for the month of October 2010 is 38,472 and for the same period the number of “Unique Visitors” is 41,240; on this specific case a difference of 6,71%. Why this happen? Is not suppose to be the same number in both places? What is the difference between “Absolute Unique Visitors” and “Unique Visitors”?
Let me explain this step by step… first at all the number of “Unique Visitors” will be ALWAYS larger that the number of “Absolute Unique Visitors”, the difference could be very small or very large depending what kind of website you have. If your site has a lot of recurring customers (banks websites – where people go on a regular basis to check balances) this difference can be pretty big, 20% or more. If your site doesn’t have any kind of recurring users this difference may be very small.
This is why…
Let’s say (as this example) you consider 1 month data range for your report, if you go to visitors (Google analytics menu on the left) you will see that” Absolute Unique Visitors” is 38,472 but if you create a custom report and add “Unique Visitors” as one of the “metrics” you will see that you will get a greater number 41,240 on this case… so which number will you believe? 38,472 or 41,240 well the truth is that both of them are accurate, here the explanation:
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“Absolute Unique Visitors” is the number of “Unique Visits” you have in that period of time, on this case Oct 2010. In other words if a user log in to your site today and then in a week and then in two weeks, that person will be counted as ONE unique visitor for the month. That’s why this number will be always the smaller one. No matter how many times a user visit your site, if they log from the same IP address they will be counted as just one visit ALWAYS.
“Unique Visitors” is the number of unique visitors cut by the day, in other words… if somebody visit your site 3 times a day for 10 days, that person will be considered ONE “Absolute Unique Visitor” for that month but 10 “Unique Visits” will be actually counted towards the “Unique Visitor” total (3 times per day = ”1 Unique Visitor a day” x 10 days). So basically what Google analytics does is that adds up Unique Visitors for the day to give you a total “Unique Visitors” for the month.