Explains the relationship between two variables only when one variable helps explain or predicts the other. Regression line: a straight line which shows a response variable y changes as the explanatory x varable changes. Used to predict the value of y for a given value of x. Increase in energy use is an explanatory variable and fat gain is a response variable. Fitting a line = line of best fit y = a + bx. Where b is the slope and a is the intercept (when x=0) is the rate of change; as x changes by one unit, y changes by the slope value y = prediction where as y = actual observation. Extrapolation: use of the regression line for prediction far outside of the range of values not so accurate because cannot say whether the graph continues to increase or decrease or whether the relationship remains linear at extreme values. Method of fitting a line to a scatter plot.