If you’re working with a large site that gets a lot of traffic or are using advanced segments, you could be running reports with a lot of sampling. Sampling is where Google basically takes a scoop of data from the whole gallon of ice cream and makes conclusions about the rest based on that scoop. (I really shouldn’t blog when I’m jonesing for a chocolate shake!)
Anyway, Google gives users an indicator that their data is being sampled, but it’s so small and subtle many miss it. So you have to train your eye to look for this symbol in the upper-right corner of your reports, right under the date range:
Google kind of slipped this little bugger into the UI quietly this past year under the guise of providing your data faster. However, moving it to the right will cut the degree of sampling applied to your data in half. So if you see it, always, always, always click it and move the slider all the way to the right like so to take some of your data back:
In the report I’m using as my example, because the report was based on 52% of visits, when I moved the slider all the way to the right, it removed sampling altogether (because doubling that takes us to 100%). And that made me feel like Robin Hood to my client. (Granted, I have a seriously over-active imagination!) In most cases you won’t remove all sampling, but you can at least double the amount of data Google uses in its sample population.