Final answer:
Probabilities assigned to experimental outcomes must add up to one, as they form a probability distribution where p + q = 1 for two possible outcomes of 'success' and 'failure'.
Step-by-step explanation:
The probabilities must be assigned to experimental outcomes so that the probabilities of all the experimental outcomes add up to one. This is because the sum of the probabilities in a probability distribution must equal one, a fundamental principle in probability theory. A probability of zero means an event will not occur, while a probability of one means an event is certain to occur. Additionally, if there are two possible outcomes, they are usually labelled as 'success' and 'failure', and the probabilities of success (p) and failure (q) must add up to one (p + q = 1).
When calculating probabilities for two mutually exclusive events, the sum rule of probability is applied. According to this rule, the probability of one event or the other occurring is the sum of their individual probabilities. This rule is essential in scenarios where there are multiple pathways to achieve an event.