Reading the notation

A small set of symbols carries every part of this paper — get them precise, since a mixed-up P(A|B) vs P(B|A) is the single most common way marks are lost.

📖 Symbols used in probability methods

6 terms

Cover the right column and check you can say what each one means.

P(A and B): The probability of both A and B happening — multiply along the branches of a tree.
P(A|B): "The probability of A GIVEN B" — restrict to the B group first, then find the proportion that is also A.
E = n × p: Expected value — the average number of successes you'd predict over n trials at probability p, over the long run.
μ, σ: The mean (μ) and standard deviation (σ) of a normal distribution — the two numbers that fully describe its shape.
z-score: z = (x − μ) / σ — how many standard deviations a value x is from the mean, used to read normal-distribution probabilities.
Relative risk: The ratio of one group's probability of an event to another group's — a value near 1 means the risk is similar; well above or below 1 means a real difference.

Before writing anything

Identify which of the three recurring question types this is: normal distribution, a probability tree, or a two-way table — each has its own checklist below.
Skim the whole question for a claim someone makes, an unknown branch to solve for, or a percentile to convert — these signal which of the deeper skill sections further down this page you will need.

Practice for this standard covers the whole topic — flashcards and quiz.