question: public HoltWintersModelBuilder alpha ( double alpha ) { this . alpha = alpha ; return this ; }
target:  Alpha controls the smoothing of the data.  Alpha = 1 retains no memory of past values (e.g. a random walk), while alpha = 0 retains infinite memory of past values (e.g. the series mean).  Useful values are somewhere in between.  Defaults to 0.5.