1. Section 7.1 ~ Seeking Correlation
Introduction to Probability and Statistics
Ms. Young ~ room 113

2. Sec. 7.1
Objective
After this section you will be able to define correlation, recognize positive and negative correlations on scatter diagrams, and understand the correlation coefficient as a measure of the strength of a correlation.

3. Sec. 7.1
Scatter Diagrams
A scatter diagram (or scatterplot) is a graph that represents the values of two variables using dots

4. Sec. 7.1
Correlation
A correlation is a relationship between two variables and can be seen easily using a scatterplot
A positive correlation occurs when both variables increase (or decrease) together
Ex. ~ Number of shoes sold and money made; the more shoes that are sold will result in more money being made
A negative correlation occurs when one variable increases while the other decreases
Ex. ~ Number of people and group rate; as the number of people increases, the group rate goes down
Ex. ~ Age and life expectancy; as you get older, your life expectancy gets lower
No correlation occurs when there is no apparent (linear) relationship between the two variables
Ex. ~ Temperature and shoe size; as your temperature increases (or decreases), your shoe size does not increase (or decrease)
A nonlinear relationship is when there is a relationship between the two variables, but not a linear one
Ex. ~ Time passed and height of a softball after being thrown straight up in the air; as the time increases, the height of the ball will increase, but then reach a maximum height and begin to decrease. This represents a quadratic relationship and would therefore be nonlinear

5. Positive Correlations
Sec. 7.1
Positive Correlations
Types of positive correlations:
Perfect Positive Correlation – when both values increase and fall in a straight line (y is directly related to x)
High (or strong) Positive Correlation – when both values increase and are tightly clustered around a straight line
Low (or weak) Positive Correlation – when both values increase and are somewhat clustered around a straight line

6. Negative Correlations
Sec. 7.1
Negative Correlations
Types of negative correlations:
Perfect Negative Correlation – when one value increases and the other decreases and they fall in a straight line
High (or strong) Negative Correlation – when one value increases and the other decreases and they are tightly clustered around a straight line
Low (or weak) Negative Correlation – when one value increases and the other decreases and they are somewhat clustered around a straight line

7. No Correlation & Nonlinear Correlations
Sec. 7.1
No Correlation & Nonlinear Correlations
Scatterplots representing no correlation
Scatterplot representing a nonlinear correlation

8. Correlation Coefficient
Sec. 7.1
Correlation Coefficient
A correlation coefficient (r) is a numerical value that represents the strength and direction of a correlation
The correlation coefficient can only range from 1 to -1, where 1 is a perfect positive correlation and -1 is a perfect negative correlation
No correlation would have a correlation coefficient close to 0
A positive correlation will only range from 0 to 1 where stronger correlations are closer to 1 and weaker correlations are closer to 0
A negative correlation will only range from 0 to -1 where stronger correlations are closer to -1 and weaker correlations are closer to 0
To calculate the correlation coefficient by hand, use the following formula:
But don’t worry, we won’t be calculating it by hand!