The match ratio describes the similarity between the users on a first-party audience list to the users on a third-party audience list. Numbers greater than 1 are a positive correlation, where higher numbers indicate a greater similarity. Match ratios under 1 correspond to a negative correlation, and generally imply a list isn't good to target.
How to interpret a list's match ratio
Generally, higher match ratios denote a greater similarity. However, when evaluating a list's match ratio, please consider the size of the list. If a list is small and has a high match ratio, it may not have enough scale to be worthwhile as compared to a larger list with a high match ratio.
Formula for calculating match ratio
Cookies on both first- and third-party lists "Probability of Overlap" ―――――――――――――――――――――――――――――――――――――――― Cookies on the third-party audience list … divided by… ―――――――――――――――――――――――――――――――――――――――――――――――――――― Cookies on the first-party audience list "Baseline" ―――――――――――――――――――――――――――――――――――――――― All possible cookies = "Match Ratio"
In other words:
The Probability of Overlap is the dividend, which is the odds of a third-party audience list cookie being a first-party audience list cookie as well.
Probability of Overlap is divided by the Baseline, which is the odds of any cookie across all inventory sources a partner has the ability to buy from being in the first-party audience list.
This equation produces the Match Ratio, which describes how much more likely it is for a member of the third-party audience list to be a member of the first-party audience list than it is for a cookie chosen at random (from all cookies across all inventory sources) to be a member of the first-party audience list.