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AP Stats



Unit 4

4.3 Introduction to Probability

1 min readjune 11, 2020

Aly Moosa

Kanya Shah

The main concepts you need to grasp from this section are the basics of probability and the complement rule. Make sure you understand these key ideas 🔐!
A probability model is a description of some chance process that is made up of two portions: a list of all possible outcomes and the probability for each outcome. The list of all possible outcomes is called a sample space.

📝Basic Probability Rules 

  1. If all outcomes in a sample space are equally likely, the probability that event A occurs is...
    1. P(A) = number of outcomes in event A/ total number of outcomes in the sample space.
  2. The probability of any event is a number between 0 and 1 (proportion).
  3. All possible outcomes MUST add up to 1
  4. The probability that an event does not occur is 1 minus the probability that it does occur. In other words, this refers to the complement rule which says that P(AC) = 1- P(A) where P(AC) is the complement of event A or the probability that event A doesn’t occur. (*Understand how to solve problems using this rule given a scenario mentioning “at least one.”)
Be sure to know how to interpret the probability you calculate. Remember to use context.


Courtesy of Make-a-Meme

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Free Response Questions (FRQs)
Unit 1: Exploring One-Variable Data
Unit 2: Exploring Two-Variable Data
Unit 3: Collecting Data
Unit 5: Sampling Distributions
Unit 6: Inference for Categorical Data: Proportions
Unit 7: Inference for Qualitative Data: Means
Unit 8: Inference for Categorical Data: Chi-Square
Unit 9: Inference for Quantitative Data: Slopes
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