Simple Rules for the Computation of Probabilities

The Additive Rule

Additive Rule: First, write down the simple events that form the event. Then, add the probabilities for each of those simple events - this total is the probability for the event.

Event Definition: E = {list of conditions} = U_i E_i = Union_i E_i

Identify simple events for the experiment: E₁, E₂, E₃, …

Note that

1 = S_{over all iPr}{E_i} = Sum_{over all i} Pr{E_i}.

Express Event as a list of simple events: "E occurs" = "One of {E_j}_j occurs"

Compute the probability for each simple event E_j. Add the simple event probabilities:

Pr{E} = S_jPr{E_j}= Sum_jPr{E_j}.