310 Chapter 10 X 3 X 4 X 5 [] = = = = population density per square mile (average = 756.2). percentage of population that is nonwhite (average = 12.5). percentage of population that is 65 or older (average = 8.4). t value. The first two explanatory variables are measures of air pollution. The last three explanatory variables are intended to accomplish the same objectives as a laboratory experiment in which these factors are held constant to isolate the effects of air pollution on mortality. With thousands of observations, the cutoff for statistical significance at the 5 percent level is a t value of approximately 1.96. The two pollution measures have substantial and statistically significant effects on the mortality rate. For a hypothetical city with the average values of the five explanatory variables, a 10 percent increase in the average suspended particulate reading, from 118.1 to 129.9, increases the mortality rate by 0.041(11.8) = 0.48, representing an additional 48 deaths annually in a city of 1,000,000. A 10 percent increase in the minimum sulfate reading, from 4.7 to 5.17, increases the mortality rate by 0.71(0.47) = 0.34 (representing an additional 34 deaths annually in a city of 1,000,000). 10.3 Multicollinearity Each coefficient in a multiple regression model tells us the effect on the dependent variable