Researchers analyzed the impact of the age of a country on the overall level of concern for the environment (Hershfield, Bang, & Weber, 2014). They noted that some countries—Sweden, for example—are more likely to enact environmentally friendly legislation, whereas others, like India, are less likely to do so. They predicted that older countries—those with more of a history—would be more likely to be future- oriented and therefore develop environmentally friendly policies. So, they conducted a regression to examine whether a country’s age predicted a country’s score on a measure called the Environmental Performance Index (EPI). They controlled for wealth, as measured by a country’s gross domestic product (GDP) and a scale that assessed a government’s overall stability. They reported that, “even after controlling for these factors, however, we found that country age accounted for approximately 6% of the variation in country-level environmental performance.” They reported a p value of 0.001. 

a. Explain how you know researchers used multiple regression in this case. 

b. What is the most likely reason that researchers controlled for wealth and stability in their multiple regression? 

c. List at least one additional variable that the researchers might have considered controlling for.