Reliability and validity of the instrument used in BRFSS to assess physical activity

State-level statistics of adherence to the physical activity objectives in Healthy People 2010 are derived from the Behavioral Risk Factor Surveillance System (BRFSS) data. BRFSS physical activity questions were updated in 2001 to include domains of leisure time, household, and transportation-related activity of moderate- and vigorous intensity, and walking questions. This article reports the reliability and validity of these questions.

The BRFSS Physical Activity Study (BPAS) was conducted from September 2000 to May 2001 in Columbia, SC. Sixty participants were followed for 22 d; they answered the physical activity questions three times via telephone, wore a pedometer and accelerometer, and completed a daily physical activity log for 1 wk. Measures for moderate, vigorous, recommended (i.e., met the criteria for moderate or vigorous), and strengthening activities were created according to Healthy People 2010 operational definitions. Reliability and validity were assessed using Cohen's kappa (kappa) and Pearson correlation coefficients.

Seventy-three percent of participants met the recommended activity criteria compared with 45% in the total U.S. population. Test-retest reliability (kappa) was 0.35-0.53 for moderate activity, 0.80-0.86 for vigorous activity, 0.67-0.84 for recommended activity, and 0.85-0.92 for strengthening. Validity (kappa) of the survey (using the accelerometer as the standard) was 0.17-0.22 for recommended activity. Validity (kappa) of the survey (using the physical activity log as the standard) was 0.40-0.52 for recommended activity.

The validity and reliability of the BRFSS physical activity questions suggests that this instrument can classify groups of adults into the levels of recommended and vigorous activity as defined by Healthy People 2010. Repeated administration of these questions over time will help to identify trends in physical activity.