But sometimes, when you modify or filter a standard report inside Google Analytics, you may notice that the numbers no longer add up.
That’s one question we get sometimes from our clients, who notice a difference between the perfected data we provide them in ChannelMix and the numbers they see in reports they create inside Google Analytics.
So why are the numbers different? Often, it’s because of sampling, a technique that Google Analytics may apply when generating on-the-fly reports.
What Is Sampling?
Google Analytics offers a great example of sampling in its Help section:
For example, if you wanted to estimate the number of trees in a 100-acre area where the distribution of trees was fairly uniform, you could count the number of trees in 1 acre and multiply by 100, or count the trees in a half acre and multiply by 200 to get an accurate representation of the entire 100 acres.
The big benefit of sampling is that it’s much faster and, if done right, still very accurate. (Though not as accurate as if — in the example above — you’d gone through and counted every single tree in the whole 100 acres, and somehow didn’t make any mistakes.)
How GA Uses Sampling
Google Analytics often uses sampling when a user modifies one of GA’s existing reports. Let’s say you apply a different segment, filter or secondary dimension. Google Analytics calls this an “ad hoc report.”
Instead of looking at every single row of data you’ve stored in Google Analytics, GA will take a percentage of those rows and extrapolate metrics for your whole dataset.
- If you’re using the standard version of Google Analytics, sampling will kick in for ad hoc reports that encompass 500,000 sessions at the view level for the date range you’re using.
- When using Google Analytics 360, sampling won’t apply until you’re dealing with 1 million to 100 million sessions at the view level for the data range you’re using. And you also have the ability to request unsampled data as a 360 client. (We’re a GA 360 reseller and can answer other questions you might have on extracting unsampled data.)
There’s an easy way to tell if your report is based on a sample. Just look in the top right hand corner of the screen. You’ll see text that reads something like “This report is based on X% of sessions.”
Next to that line, you’ll see a toggle button that gives you the ability to control your report’s sample size. You can choose between “Greater precision” and “Faster response.”
And if you go back and remove that filter, dimension or segment you applied to the default report, you’ll see that your numbers are back to being based on 100 percent of your rows.
Need help with your Google Analytics? Let’s talk!