AP Statistics
3.1 Scatterplots
Objectives: Explain the difference between an explanatory variable and a response variable.
Given a set of bivariate data, construct a scatterplot.
Explain what is meant by the direction, form, and strength of the overall pattern of a
scatterplot.
Explain how to recognize an outlier in a scatterplot.
Explain what it means for two variables to be positively or negatively associated.
Explain how to add categorical variables to a scatterplot
Use a TI-83/89 to construct a scatterplot.
The Relationship of Two Variables
 Remember that the relationship between two variables can be STRONGLY
influenced by other variables that are lurking in the background.
 Remember W5HW – Who, What, Why, When, Where, How, and by Whom.
Response Variables (y) measures an outcome of a study.
Explanatory Variables (x) helps explain or influence change.
REMEMBER that calling one variable explanatory and the other response DOES NOT
necessarily mean that changes in one causes changes in the other.
Exercise 3.1 (pg. 173)
a)
b)
c)
d)
e)
Scatterplot: A scatterplot shows the relationship between two quantitative variables measured on
the SAME individual. The values of the eXplanatory variable appear on the horizontal
axis and the response appears on the vertical axis. Each individual in the data appears
as a point in the plot fixed by the values of both variables for the individual.
KEYS TO A SCATTERPLOT
 Title
 Label your axes
 Keep scale uniform
 Plot your points so you can read the pattern easy.
Interpreting Scatterplots
 Pattern: Look for an overall pattern and look for striking deviations.
o Direction, Form and Strength: Describe the pattern with the following terms:
Direction: How does the pattern move from left to right?
Form: Is the pattern curved, or a straight line?

Strength: Are the points close to the form?
YES  Strong
NO  Weak
Outliers: An important kind of deviation is an outlier – an individual value that falls outside of
the overall pattern.
Positive Association  A pattern that RISES from left to
right – starts in the lower left corner and rises to the upper
right corner.
Negative Association  A pattern that FALLS from left to right – starts in the upper left corner and
falls to the lower right corner.
Categorical Variables in Scatterplots  To add a
categorical variable to a scatterplot, use a different color
(see figure 3.3 pg. 179)
Exercise 3.5 (pg. 179)
SCATTERPLOT NOTES ON THE GC
HW: pg. 173; 3.2 – 3.4 and pg.179; 3.6 – 3.8