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wpolyfit

PURPOSE ^

% Return the coefficients of a polynomial @var{p}(@var{x}) of degree

SYNOPSIS ^

function [p_out, s, mu] = wpolyfit (varargin)

DESCRIPTION ^

% -*- texinfo -*-
% @deftypefn {Function File} {[@var{p}, @var{s}] =} wpolyfit (@var{x}, @var{y}, @var{dy}, @var{n})
% Return the coefficients of a polynomial @var{p}(@var{x}) of degree
% @var{n} that minimizes
% @iftex
% @tex
% $$
% \sum_{i=1}^N (p(x_i) - y_i)^2
% $$
% @end tex
% @end iftex
% @ifinfo
% @code{sumsq (p(x(i)) - y(i))},
% @end ifinfo
% to best fit the data in the least squares sense.  The standard error
% on the observations @var{y} if present are given in @var{dy}.
%
% The returned value @var{p} contains the polynomial coefficients 
% suitable for use in the function polyval.  The structure @var{s} returns
% information necessary to compute uncertainty in the model.
%
% To compute the predicted values of y with uncertainty use
% @example
% [y,dy] = polyconf(p,x,s,'ci');
% @end example
% You can see the effects of different confidence intervals and
% prediction intervals by calling the wpolyfit internal plot
% function with your fit:
% @example
% feval('wpolyfit:plt',x,y,dy,p,s,0.05,'pi')
% @end example
% Use @var{dy}=[] if uncertainty is unknown.
%
% You can use a chi^2 test to reject the polynomial fit:
% @example
% p = 1-chisquare_cdf(s.normr^2,s.df);
% @end example
% p is the probability of seeing a chi^2 value higher than that which 
% was observed assuming the data are normally distributed around the fit.
% If p < 0.01, you can reject the fit at the 1% level.
%
% You can use an F test to determine if a higher order polynomial 
% improves the fit:
% @example
% [poly1,S1] = wpolyfit(x,y,dy,n);
% [poly2,S2] = wpolyfit(x,y,dy,n+1);
% F = (S1.normr^2 - S2.normr^2)/(S1.df-S2.df)/(S2.normr^2/S2.df);
% p = 1-f_cdf(F,S1.df-S2.df,S2.df);
% @end example
% p is the probability of observing the improvement in chi^2 obtained
% by adding the extra parameter to the fit.  If p < 0.01, you can reject 
% the lower order polynomial at the 1% level.
%
% You can estimate the uncertainty in the polynomial coefficients 
% themselves using
% @example
% dp = sqrt(sumsq(inv(s.R'))'/s.df)*s.normr;
% @end example
% but the high degree of covariance amongst them makes this a questionable
% operation.
%
% @deftypefnx {Function File} {[@var{p}, @var{s}, @var{mu}] =} wpolyfit (...)
%
% If an additional output @code{mu = [mean(x),std(x)]} is requested then 
% the @var{x} values are centered and normalized prior to computing the fit.
% This will give more stable numerical results.  To compute a predicted 
% @var{y} from the returned model use
% @code{y = polyval(p, (x-mu(1))/mu(2)}
%
% @deftypefnx {Function File} wpolyfit (...)
%
% If no output arguments are requested, then wpolyfit plots the data,
% the fitted line and polynomials defining the standard error range.
%
% Example
% @example
% x = linspace(0,4,20);
% dy = (1+rand(size(x)))/2;
% y = polyval([2,3,1],x) + dy.*randn(size(x));
% wpolyfit(x,y,dy,2);
% @end example
%
% @deftypefnx {Function File} wpolyfit (..., 'origin')
%
% If 'origin' is specified, then the fitted polynomial will go through
% the origin.  This is generally ill-advised.  Use with caution.
%
% Hocking, RR (2003). Methods and Applications of Linear Models.
% New Jersey: John Wiley and Sons, Inc.
%
% @end deftypefn
%
% @seealso{polyfit,polyconf}

CROSS-REFERENCE INFORMATION ^

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