You are measuring the concentration of stress hormone cortisol in saliva samples from volunteers and want to determine whether those who are parents have higher cortisol levels than non-parents. Which of these, if true, would be confounding factors in your study? You can choose more than one. Parents tend to be older, and older people are less stressed. There is no difference in stress levels between parents and nonparents. You have measured only a very small number of parents. You have measured only a very small number of non-parents. Parents tend to be older, and older people are more stressed.