According to the U.S. Census Bureau, in 2009, 10% of adults 25 years and older in the United States had advanced degrees. A researcher with the U.S. Department of Education surveys 500 randomly selected adults 25 years of age or older and finds that 60 of them have an advanced degree. Explain why this is not necessarily evidence that the proportion of adults 25 years of age or older with advanced degrees has increased.