Signal helps users answer the questions “why do customers retain in my product?” and “which events are associated with a higher retention?” Signal does this by measuring and evaluating the association between your events and the goal event that you care about.
Signal calculates the association using a well-known statistical algorithm called the phi coefficient and then supplements with an evaluation of how actionable this information is using our own measurements for opportunity. The phi coefficient is a single number between -1 and 1 that indicates how closely an event moves with your goal event. 1 means that 100% of all users that did your goal event also did the event you are evaluating. -1 means the opposite: 0% of users that did you goal event also did the event you are evaluating.
Related article: When should I use Signal?