[Update: May 10th - This warning originally read "This report is based on sampled data". Google have now explained that the warning appears any time you report on more than one dimension in Analytics on a date range containing more than 500,000 visits. See the Google Analytics Fast Access Mode help page for more detail.]
Everyone loves Google Analytics. It’s free, full of great features, and most of the time it works like a charm. We use GA data regularly in WhatClinic.com to make important decisions about everything from internal linking structures to on page keyword priorities, and it really comes into its own when something on the site is broken and we need to track the problem down.
However, like a lot of great free web services GA suffers from being too popular. The amount of data it has to crunch for our website alone is staggering, so imagine the load when you add in the tens if not hundreds of millions of other websites that use it too. Unfortunately the strain starts to show as soon as you really dig into your data.
The Dreaded Fast Access Mode Message
One of my most used features on GA is the Advanced Segment, which lets me filter the traffic I’m reporting on in a myriad of different ways. I can report on only the people who landed on our search pages, or only the people who came from Canada, or pretty much any set of people with an analytics parameter in common.
Google have a problem with this great feature though: in order to give you an answer to your query in a reasonable length of time they often fall back on using sampled data, or “Fast Access Mode” as they now call it, if you are using a date range that contains over 500,000 visits.
Sampled Data
Using sampled data is absolutely fine when your data points don’t suffer from a lot of volatility, but if they do then the results can be very unreliable. Take a look at the data set above. It uses sampled data because I’ve created an Advanced Segment with two parameters (user location and page URL) and I’m looking at a period of three and a half months which contains far more than 500,000 visits. Analytics is reporting a conversion rate in April of just 2.65% for the set of visitors I’m looking at.
Now look at the data set below. It is reporting a conversion rate in April of 4.36%. The difference is that it isn’t looking at sampled data because the period of time is much shorter and the visitors number is less than 500,000.
Real Data
Our conversion data can be very volatile in places because of the niche or long tail nature of what we offer and what I need to report on from time to time. For instance. We might get 1,000 enquiries for dental clinics in Dublin in a week, but if I look at just the visitors who were looking for clinics in Rathmines that figure might drop to just 10, and those ten might have been made up of 3 on Monday, none on Tuesday, 1 on Wednesday, and so on. It’s the same when you look at American visitors to our Mexican dentists pages, or UK visitors to our Turkish cosmetic surgery pages.
Be Sure To Perform A Sanity Check
As with the case above, when something looks like a problem we all tend to look into it and see what’s going on, but when something looks good we tend to let it slide. My advice is any time you see the Fast Access Mode message in GA you should sanity check it by using shorter periods of time. This takes a little time but it will give you a far more reliable picture of what’s going on.
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