Ensemble Kalman Filtering is efficient because matrix operations are performed in the low-dimensional space of the ensemble perturbations
						Ensemble Kalman Filter Analysis: correction computed in the low dim attractor
	Observations ~105-7 d.o.f.
							Background ~106-8 d.o.f.
							Errors of the day: they lie on the low-dim attractor
			3D-Var Analysis: doesn’t know about the errors of the day