Examining Relationships Chapter 3

Response Variable Measures an outcome of a study Dependent

Explanatory Variable Attempts to explain the observed outcomes Independent

Scatterplots Section 3.1

Scatterplot Most effective way to display the relation between two quantitative variables measure on the same individuals.

Explanatory variable plotted on the x-axis Response variable plotted on the y-axis Categorical variables require different symbols.

Examining a Scatterplot Look for an overall pattern and deviations from that pattern Describe the pattern by the form, direction, and strength of the relationship

Positive association One variable increases so does the other variable. One variable decreases so does the other variable. Similar to positive slope.

Negative association One variable increases and the other variable decreases, or vice versa. Similar to negative slope.

Correlation Section 3.2

Measures the direction and strength of the linear relationship between two quantitative variables Usually written as r

Facts About Correlation Makes no distinction between explanatory and response variables Requires both variables to be quantitative Has no unit of measure…is just a number

Correlation is always between -1 and 1 Correlation only measures the strength of linear relationships Correlation is not resistant Correlation is not a complete description of two-variable data—always give mean and standard deviation as well

Practice Problems pg. 122 #3.1-3.4 pg. 135 #3.15-3.23