Joint probability
SAT glossary · Math
Definition
P(A and B) — the count in both categories over the grand total, e.g. 45/100 = 0.45 for 'studied and passed.' It is the answer to a question with no 'given' in it, and it is offered as a distractor to students who spot the cell count and divide by the grand total out of habit.
Joint probability in plain English
Joint probability is the chance that TWO things both happen at the same time — written P(A and B) — and on a two-way table, you find it by taking the count of people/things in BOTH categories at once and dividing by the grand total of everyone. For example, if 45 out of 100 people 'studied AND passed,' that's 45/100=0.45. Notice there's no 'given' language here — you're not narrowing down to a smaller group first, you're comparing straight to the whole population. A common trap: students who correctly spot the right cell in the table sometimes divide by the grand total out of habit even when the question actually asked for a CONDITIONAL probability (which needs a row or column total instead) — so always double check which denominator the specific question wants.
Learn these first
Sign up free: all 157 SAT terms as flashcards, plus practice questions with full explanations →