a 24 hour study on myself (n = 1) during which I wore an Apple Watch 3 on my left wrist, Fitbit Charge 2 on my right wrist, and a 7-lead ambulatory electrocardiogram and went about my day. This resulted in ~3,600 heart rate observations across devices.
Fitbit Charge 2 and Apple Watch 3 Scatterplot
Correlations Between Devices
Fitbit and Apple (r = .767, p < 2.2e-16, 95% CI [.732, .799])