Smoothing parameter ($\lambda$)

A smoothing parameter controls the amount of averaging or regularization in a fitted function. Increasing the penalty multiplier $\lambda$ in a cubic smoothing spline discourages curvature and generally reduces effective degrees of freedom. Increasing a smoothing bandwidth enlarges local averaging neighbourhoods. Software may use a monotone transformed parameter such as spar, so its numerical value should not be confused with the raw penalty multiplier.