. Similarly, it would be a bit overzealous to hold age constant by requiring all participants to have been born on June 13, 1992. Instead, researchers often choose to restrict a variable to a limited range instead of holding it absolutely constant. For example, a researcher may require partic- ipants to be between 18 and 21 years of age and to have IQ scores between 100 and 110. Although age and IQ are not perfectly constant here, the restricted range should ensure that the participants in one treatment are not noticeably older or smarter than the participants in another treatment.