1 Exploring Data

Individuals and variables
- Individuals are the objects described by a set of data. Individuals may be people, animals, or things.
- A variable is any characteristic of an individual. A variable can take different values for different individuals.
Categorical variable and quantitative variable
- A categorical variable places an individual into one of several groups or categories.
- A quantitative variable takes numerical values for which it makes sense to find an average.
Distribution
- The distribution of a variable tells us what values the variable takes and how often it takes these values.

1.1 Analyzing Categorical Data

Two-Way Tables

This is a two-way table because it describes two categorical variables, gender and opinion about becoming rich.

Young Adults by Gender and Chance of Getting Rich
Opinion	Female	Male	Total
Almost no chance	96	98	194
Some chance but probably not	426	286	712
A 50-50 chance	696	720	1416
A good chance	663	758	1421
Almost certain	486	597	1083
Total	2367	2459	4826

Marginal distribution
- The marginal distribution of one of the categorical variables in a two-way table of counts is the distribution of values of that variable among all individuals described by the table.
Conditional Distribution
- A conditional distribution of a variable describes the values of that variable among individuals who have a specific value of another variable.
- There is a separate conditional distribution for each value of the other variable.
Association
- We say that there is an association between two variables if knowing the value of one variable helps predict the value of the other.
- If knowing the value of one variable does not help you predict the value of the other, then there is no association between the variables.