Receiver operator characteristic curves for classifying activity in pregnant women. Assessing moderate-to-vigorous physical activity (MVPA) using receiver operator characteristic (ROC) curves for two modalities (PAR = physical activity record, SWA = SenseWear armband). Sensitivity and specificity were determined through the entire range of cut-off times (1-1400 minutes) by comparing classification as an exerciser/non-exerciser determined by the modalities against the reference standard. "Random guess" is the line that would be obtained if classifications were made at random. AUC = area under the curve; 0.500 the AUC of a random guess.