Table 1. Sample selection in each data source.

	Tadalafil DMD Trial	Leuven	CCHMC
Step 1: All patients	116	158	480
Step 2: Patients with at least 2 4SC visits	116	90	261
Step 3: Patients with 4SC visits approximately 1-year apart (and total # of such intervals)	111 (111)	70 (1011)	230 (877)
Step 4: Patients and intervals meeting eligibility criteria and having non-missing data on prognostic factors at baseline visit of the interval	92 (92)	67 (711)	213 (684)
Step 5: Patients in Step 4 with non-overlapping intervals	92 (92)	67 (235)	212 (543)

4SC, 4-stair climb; CCHMC, Cincinnati Children's Hospital Medical Center; DMD, Duchenne muscular dystrophy.

Steps 3, 4, and 5 show number of patients (intervals) meeting each criterion. In step 3, for the Leuven and CCHMC databases, ~1-year intervals were identified on the basis of having 4SC visits 8–16 months apart. For the Tadalafil DMD trial, the ~1-year intervals reflect the ~1-year between the baseline and 48-week assessments in the trial.