"Absolute Unique" and "Unique" Visitors in Google Analytics/
July 30, 2010
Recently several people asked me about confusing numbers involving unique visitors (one was a customer and the other used the great little blog idea submission form at the right).
Visitors represent the number of distinct people who come to your site (rather than Visits, which is the number of times that they come). Seems pretty straightforward, right? Well…
One person wrote:
I’ve been puzzled about the difference between “Absolute Unique Visitors” and “Unique Visitors”. In some cases, # of Unique Visitors is double of Absolute Unique Visitors! How it could be? I have no idea how Unique Visitors is calculated and its definition.
And the other asked:
How much variability is there in google analytics? When I run a custom report to get unique visitors over a month it can range from 195k to 214k. Sometimes I get a daily count of 0 and other times I get a count of 261 for the same day.
There are potentially several issues going on here.
Two kinds of “unique” visitors
First of all, as the first question points out, there are actually two metrics for unique visitors in Google Analytics. There’s Absolute Unique Visitors and Unique Visitors.
Absolute Unique Visitors is the metric you get in the report of the same name in the Visitors section. It used to be the only place you could get visitor metrics in GA. It’s also fairly limited: notice that you can’t use Advanced Segments on this report (one of the few where you can’t).
Unique Visitors is another metric that was introduced for use in Custom Reports (which you can use Advanced Segments on as well). It was an update to the Absolute Unique Visitors that was more compatible with these new features, basically.
The metrics themselves actually differ slightly in the way they measure visitors (how they tabulate the individual cookies). So the numbers won’t match up exactly. In fact, Unique Visitors is actually more accurate than Absolute Unique Visitors (surely a nominee for the Worst Title Ever in Google Analytics). Unique Visitors will usually show a slightly greater number (on the order of 5-20%, based on a few sample accounts I looked at). Since Unique Visitors is the more accurate and flexible metric, Absolute Unique Visitors is being phased out over time and someday we will have the One True Visitor Metric.
Uniques depend on the time period
One really important thing to recognize is that, “unique” depends on what time period we are referring to.
Take this Absolute Unique Visitors report.
For the 30 days that I am looking at here, there were 12,926 Absolute Unique Visitors (listed at the top there). But if I went through and added up the numbers for each of those 30 days (in the bar chart), it wouldn’t equal 12,926. Why?
Well, suppose I visit the website on Monday. Maybe I visit once or twice or 67 times, it doesn’t matter. I am 1 unique visitor for Monday, and the bar for Monday goes up by 1.
Now I visit again on Tuesday (once or twice or 67 times again). Tuesday goes up by 1.
BUT, for the whole month, I am still just 1 unique visitor out of those 12,926. It doesn’t matter whether I only came once on one particular day during that period, or once every day, or 67 times every day.
So it’s really important that you recognize this when you talk about unique visitors. Are these daily unique visitors? Monthly? It’s really important what the time frame is. You can’t, for example, take the number of monthly unique visitors for January, February, …, December and just add them up to get the annual unique visitors.
The dreaded sampling
The data in the standard Google Analytics reports is all pre-computed, and it’s pretty exact. If a visit got recorded, you’re going to see it there.
However, when you start combining things that weren’t pre-computed — for example, with an Advanced Segment or Custom Report — Google Analytics does the computations on the fly. If you have a really busy site, this would potentially take a looooooong time.
So instead, if you have more than 500,000 visits in the time period you are looking at and you request data that needs to be computed on the fly, Google Analytics takes a statistical sample of the data and gives you an estimate with a range of error, rather than tabulating every single visit. You’ll get a little notice that says “this report is based on sampled data” and a +/- error margin along with your metrics. (There’s more info about how this works in the Google Analytics documentation.)
This is something you should also be aware of when you’re looking at reports, because it can drastically affect accuracy, especially for segments or dimensions where the total number of visits is small compared to the total visits. If you need exact numbers, try to keep to time periods with fewer than 500,000 visits.