1. Correlation Coefficient
“r-value”

2. Scatterplots and Correlation
Measuring Linear Association: Correlation
A scatterplot displays the strength, direction, form and outliers of the relationship between two quantitative variables.
Unfortunately, our eyes are not good judges of how strong a relationship is.
Linear relationships are important because a simple linear pattern is quite common.
Scatterplots and Correlation

3. Correlation Coefficient
The correlation coefficient computed from sample data measures the strength and direction of a linear relationship between two quantitative variables.
Definition:
The correlation r measures the strength of the linear relationship between two quantitative variables.
r is always a number between -1 and 1
r > 0 indicates a positive association.
r < 0 indicates a negative association.
Values of r near 0 indicate a very weak linear relationship.
The strength of the linear relationship increases as r moves away from 0 towards -1 or 1.
The extreme values r = -1 and r = 1 occur only in the case of a perfect linear relationship.

4. Measuring a Linear Correlation
Remember…
r is a value between -1 and 1.
The closer it is to -1 or 1, the stronger the relationship
Strong < Weak---NoCorr----Weak > Strong
R value (correlation coefficient)
No Correlation

5. Other Examples of Approximate r-valuesy y y x x x
r = -1
r = -.6
r = 0 y y x x
r = +.3
r = +1

6. Scatterplots and Correlation
Facts about Correlation
How correlation behaves is more important than the details of the formula. Here are some important facts about r.
Scatterplots and Correlation
Correlation makes no distinction between explanatory and response variables. (you get the same answer even if you switch the x and y)
r does not change when we change the units of measurement of x, y, or both.
The correlation r itself has no unit of measurement.
Cautions:
Correlation requires that both variables be quantitative.
Correlation does not describe curved relationships between variables, no matter how strong the relationship is.
Correlation is not resistant. r is strongly affected by a few outlying observations.
Correlation is not a complete summary of two-variable data.

7. Scatterplots and Correlation
The formula for r is a bit complex. It helps us to see what correlation is, but in practice, you should use your calculator or software to find r.
Scatterplots and Correlation
How to Calculate the Correlation r
Suppose that we have data on variables x and y for n individuals.
The values for the first individual are x1 and y1, the values for the second individual are x2 and y2, and so on.
The means and standard deviations of the two variables are x-bar and sx for the x-values and y-bar and sy for the y-values.
The correlation r between x and y is:

8. Match the Correlation Coefficient to the graph
Correlation Coefficients

9. Match the Correlation Coefficient to the graph
Correlation Coefficients

10. Match the Correlation Coefficient to the graph
Correlation Coefficients

11. Match the Correlation Coefficient to the graph
Correlation Coefficients

12. Match the Correlation Coefficient to the graph
Correlation Coefficients

13. Scatterplots and Correlation
Correlation Practice
For each graph, estimate the correlation r and interpret it in context.
Scatterplots and Correlation

14. Put the correlation coefficients in order from weakest to strongest
Ex 1: 0.87, -0.81, 0.43, 0.07, Ex 2: 0.32, -0.65, 0.63, -0.42, 0.04
0.07, 0.43, -0.81, 0.87, & -0.98
0.04, 0.32, -0.42, 0.63, & -0.65

15. Scatterplots and Correlation
Summary
In this section, we learned that…
The correlation r measures the strength and direction of the linear relationship between two quantitative variables.