Classification tree of National Health and Nutrition Examination Survey physical function questions using the presence of a 2-min bout of moderate-vigorous physical activity as the outcome variable. The first split is for participants who responded they had no difficulty walking up 10 steps, of which 78.9% had a 2-min bout of moderate-vigorous physical activity. The sensitivity of detecting a 2-min bout of activity increased further when participants answered they had no difficulty walking up 10 stairs and were able to walk two to three blocks. The classification tree was unable to further improve prediction of the outcome variable with additional questions for participants who answered they had some or much difficulty or unable to walk upstairs. Questions: (Stairs) “By yourself and without using any special equipment, how much difficulty do you have walking up 10 steps without resting?” (Walk two to three blocks) “By yourself and without using any special equipment, how much difficulty do you have walking for a quarter mile [that is about 2 to 3 blocks]?” Possible responses to the physical function questions: (1) No difficulty; (2) Some difficulty; (3) Much difficulty; (4) Unable to do; or (5) Do not do this activity. LR+, positive likelihood ratio; LR−, negative likelihood ratio; MVPA, moderate-vigorous physical activity.