The backfitting algorithm estimates each additive mean function by smoothing its partial residual, obtained after subtracting the current fits of all other terms, and cycling through terms until convergence. Centering each smooth separates its constant part from the intercept. For fixed quadratic smoothness penalties it is block coordinate minimization of a penalized least-squares objective; identifiability and positive definiteness of the constrained problem ensure a unique converged fit. Non-Gaussian generalized additive models can use weighted backfitting inside iteratively reweighted least squares.
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Backfitting is an iterative algorithm used primarily in the context of fitting additive models, particularly generalized additive models (GAMs). An additive model assumes that the response variable can be expressed as a sum of smooth functions of predictor variables. The backfitting algorithm helps to estimate the smooth functions in such models.