Sampling is a process used in statistics when it's unfeasible or impractical to analyse all the data that exists. Instead, a small, randomly selected subset is used to keep things manageable. Many analytics platforms use some sort of sampling to keep report loading times in check, and there seem to be three schools of thought when it comes to sampling in analytics. There are those who are terrified of it, insisting in unsampled versions of any report. Then there are those who are relaxed about it, trusting the statistical logic. And then, lastly, there are those who are oblivious.
All three are misguided.
Sampling isn't something to fear, but, in Google Analytics in particular, it can't always be trusted. Because of that, it's definitely worth your time to understand when it occurs, how it affects your work, and how it can be avoided.
When it happens-
The exact same level of sampling can also be induced through use of a secondary dimension:
Secondary dimension applied, report based on 0.17% of sessions.
A few other specialised reports are also prone to this level of sampling, most notably:
- The Ecommerce Overview
- "Flow Reports"
Report based on <0.1% of sessions
To summarise so far, sampling can happen when we use:
- A segment
- More than one dimension
- Certain detailed reports (including Ecommerce Overview and AdWords Campaigns)
- "Flow" reports
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