This has to do with the union of events. If events A and B are in the set S, then the union of A and B is the set of outcomes in A or B. This means that either event A or event B, or both, can occur.
P(A or B) = P(A) + P(B) - P(A and B)
**P(A and B) is subtracted, since by taking P(A) + P(B), their intersection, P(A and B), has already been included. In other words, if you did not subtract it, you would be including their intersection twice. Draw a Venn Diagram to visualize.
If A and B can only happen separately, i.e., they are independent events and thus P(A and B) = 0, then,
P(A or B) = P(A) + P(B) - P(A and B) = P(A) + P(B) - 0 = P(A) + P(B)
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A probability sample is one in which each member of the population has the same probability of being included. An alternative and equivalent definition is that it is a sample such that the probability of selecting that particular sample is the same for all samples of that size which could be drawn from the population.
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Theoretical probability is the probability of an event when all outcomes are equally likely. With theoretical probability, you determine the probability by dividing the number of ways the event can occur by the total number of equally likely outcomes.
http://www.emathzone.com/tutorials/basic-statistics/definition-of-probability.html
The experimental probability, by definition, can only be determined after you have carried out the experiment!