Probability Basics

Manual: General · Subject: Statistics

Learn the first ideas of probability and how chance connects to statistics.

Chance and uncertainty

Probability measures how likely an event is to happen. It ranges from $0$ to $1$ , where $0$ means impossible and $1$ means certain.

For equally likely outcomes, $P(\text{event}) = \frac{\text{number of favorable outcomes}}{\text{total number of outcomes}}$

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Probability and data

Statistics uses probability to model randomness and to make predictions from samples.

Simple outcomes

Event	Probability
Getting heads on a fair coin	$1/2$
Rolling a 6 on a fair die	$1/6$
Getting an even number on a fair die	$3/6 = 1/2$

What is the probability of rolling a 6 on a fair six-sided die?

1/2

1/3

1/6

1/12

What does a probability of 0 mean?

Which probability value represents certainty?

0 0.25 0.5 1

Why is probability important in statistics?