How do you find the theoretical probability of an event?
This means that you cannot calculate a probability (even theoretically) unless you have data containing BOTH types of outcomes.
You must also ALWAYS consider the “Confidence Interval” defined by your data! The math will always produce a number, but the use of that number requires understanding and wisdom. A small dataset will have a high level of error associated with it. A large dataset may not yet be available.
The math gets more involved as we apply it to different types of data, conditions, and desired outcomes, but it is always some form of this basic relationship.