Google Analytics works fine for small and medium-sized websites, but for accounts with big data volumes Google uses sampling to make the data processing faster. The default Google Analytics reports are always unsampled. Sampling takes effect when a custom filter, advanced segment or secondary dimension is used, or a custom report is created. The sampling begins when there are more than 500,000 visitors for the selected time range. Keep in mind though that the amount of visitors is calculated on the web property level, not on the profile level. That means that in profiles that represent only a small percentage of the total web property traffic, sampling can be extreme and the confidence for the data reliability low.
See in the following presentation some of the most common issues related to sampling in Google Analytics and how you can overcome them.