Complete the sentence below. In a probability model, the sum of the probabilities of all outcomes must equal (Type an integer or a decimal. Do not round.)

Step 1 ：In a probability model, the sum of all probabilities of all outcomes must equal 1. This is because the probability of an event is defined as the number of ways an event can occur divided by the total number of outcomes. Therefore, when we sum up the probabilities of all possible outcomes, it must equal 1.

Step 2 ：The sum of the probabilities of all outcomes in a probability model must equal \(\boxed{1}\).