Chapter 4 – SOAN 3130 – Week 2
- response variable
o measures an outcome of a study
o also known as dependent variables
- explanatory variable
o may explain or influence changes in a response variables
o also known as independent or predictor variables
Example 4.1 Beer and Blood Alcohol
- how does drinking beer affect the level of alcohol in our blood? The legal
limit for driving in all states is 0.08%. Student volunteers at the Ohio state
University drank different numbers of cans of beer. Thirty minutes later, a
police officer measured their blood alcohol content. Number of beers
consumed is the explanatory variable, and the percent of alcohol in the blood
is the response variable.
Example 4.2 College Debts
- A college student aid officer looks at the findings of the National Student
Loan Survey. She notes data on the amount of debt of recent graduates, their
current income and how stressed they feel about college debt. She isn’t
interested in predictions but is simply trying to understand the situation of
recent college graduates. The distinction between explanatory and response
variables does not apply. A sociologist looks at the same data with an eye to
using amount of debt and income, along with other variables, to explain the
stress caused by college debt. Now amount of debt and income are
explanatory variables and stress level is the response variable.
Displaying Relationships: Scatterplots
The most useful graph for displaying the relationship between two quantitative
variables is a scatterplot. A scatterplot shows the relationship between two
quantitative variables measured on the same individuals. One variable goes on the
horizontal axis and the other on the vertical.
How to label the scatterplot: Always plot the explanatory variable (if there is one)
on the horizontal axis f a scatterplot. This is variable “x” (the explanatory variable)
while the response variable is y. If there is no distinction between the two variables,
either variable can go on the horizontal axis.
- look for the overall pattern and for striking deviations from that pattern.
- You can describe the overall pattern of a scatterplot by the direction, form,
and strength of the relationship.
- An important kind of deviation is an outlier.
o Example: the overall pattern moves from the upper left to the lower
right. This is known as a negative association between the two
variables. Chapter 4 – SOAN 3130 – Week 2