10.39. A study of compulsive buying behavior conducted a national telephone survey in 2004 of adults ages 18 and over.7 The study found that lower income subjects were more likely to be compulsive buyers. They reported "Compulsive buyers did not differ significantly from other respondents in mean total credit card balances, but the compulsive buyers' lower income was a confounding factor." Explain what it means to say that income was a confounding factor, and explain why a comparison of the mean total credit card balances between compulsive and noncompulsive buyers could change depending on whether income is controlled.