Characteristics of study populations across exam years.

Values are mean ± standard error. Data are from cross-sectional nationally representative samples of adults (≥19 y) taken from NFCS 1977–78, CSFII 1989–91 and 1994–98, and NHANES 2003–06.aValues are different from 1977–78, pt test.bValues are different from 1989–91, pt test.cValues are different from 1994–96, pt test.dValues are standardized to the age, race, and gender distributions of 1977–78 sample population using predicted means. Predicting violates the assumption of independence required for performing Student's t tests of means; therefore, statistically significant differences are not calculated for these measures.BA, Bachelor of Arts; GED, General Equivalency Diploma.