Unique visitors
Unique visitors are visitors that we identify as different actual human beings. While our methods are proprietary and there's no single full proof method, our estimates should be within 1% of the actual number of unique visitors.
When the same user visits your site more than once in different time periods, that user will be counted once on each of these time periods individually, but will also only count once in a larger time period that includes both time periods
For example:
Day 1: 50 unique visitors
Day 2: 50 unique visitors
During these 2 days, some of the visitors from Day 1 also visited your site on Day 2.
If there is an overlap of 20 visitors (meaning 20 visitors visited your site both on Day 1 and on Day 2), then the total unique visitors during these two days would be 80 unique visitors.
Discrepancies between platform
When comparing the number of unique visitors calculated by different platforms (e.g. MixPanel, Google Analytics, etc.) the numbers might not perfectly match.
In most scenarios the difference in numbers is minor, but it might become substantial due to several factors:
Session length - when a platform shows the number of non-unique visitors, and there are visitors who visited the site multiple times, the distinction between different visits isn't set in stone. While it's usually 15 minutes, there are many ways to measure that.
Uniqueness - each platform has its own criteria for measuring uniqueness. This might include cross device synchronization in some cases, IP address, and other heuristics performed by the different platforms.
Time period - each platform updates its data at a different rate. This means that the data on one platform can be more up to date than the data on another.
Tracking mechanism - some browsers can be set up to block some of the measurement techniques. This might cause platforms that rely on a specific technique to show less visitors.