### correlation

measure the direction and strength of the linear relationship between two quantitative variables.

### residual plot

scatterplot of the residuals against the explanatory variable. help asses how well a regression line fits the data.

### residual

difference between an observed data value of the response variable and the value predicted by the regression line.

### extrapolation

use of a regression line for prediction far outside the interval of values of the explanatory variable x used to obtain the line.

### regression line

line that describes how a response variable y changes as an explanatory x changes. we use it to predict the value of y for a given value of x.

### negative association

when above average values of one tend to accompany below average values of the other.

### positive association

above average values of one tend to accompany above average values of the other, and when below values tend to occur together.

### scatterplot

shows the relationship between two quantitative variables measured on the same individuals. data appears as a point on the graph.

### observational study

observes individuals and measures variables of interest but does not attempt to influence the responses.

### available data

data that were produced in the past for some other purpose but that may help answer a present question.