Numerical recipes

15.4 General Linear Least Squares 673
a vector of length M.
The equations (15.4.6) or (15.4.7) are called the normal equations of the leastsquares
problem. They can be solved for the vector of parameters a by the standard
methods of Chapter 2, notably LU decomposition and backsubstitution, Choleksy
decomposition, or Gauss-Jordan elimination. In matrix form, the normal equations
can be written as either
[α] · a =[β] or as
The inverse matrix Cjk ≡ [α] −1
jk
� A T · A � · a = A T · b (15.4.10)
is closely related to the probable (or, more
precisely, standard) uncertainties of the estimated parameters a. To estimate these
uncertainties, consider that
aj =
M�
[α] −1
k=1
M�
jk βk = Cjk
k=1
� N�
i=1
yiXk(xi)
σ2 �
i
(15.4.11)
and that the variance associated with the estimate aj can be found as in (15.2.7) from
σ 2 (aj) =
N�
i=1
Note that αjk is independent of yi, so that
Consequently, we find that
σ 2 (aj) =
M�
M� ∂aj
=
∂yi
M�
k=1 l=1
CjkCjl
σ 2 i
� ∂aj
∂yi
� 2
CjkXk(xi)/σ
k=1
2 i
� N�
i=1
Xk(xi)Xl(xi)
σ2 �
i
(15.4.12)
(15.4.13)
(15.4.14)
The final term in brackets is just the matrix [α]. Since this is the matrix inverse of
[C], (15.4.14) reduces immediately to
σ 2 (aj) =Cjj
(15.4.15)
In other words, the diagonal elements of [C] are the variances (squared
uncertainties) of the fitted parameters a. It should not surprise you to learn that the
off-diagonal elements Cjk are the covariances between aj and ak (cf. 15.2.10); but
we shall defer discussion of these to §15.6.
We will now give a routine that implements the above formulas for the general
linear least-squares problem, by the method of normal equations. Since we wish to
compute not only the solution vector a but also the covariance matrix [C], it is most
convenient to use Gauss-Jordan elimination (routine gaussj of §2.1) to perform the
linear algebra. The operation count, in this application, is no larger than that for LU
decomposition. If you have no need for the covariance matrix, however, you can
save a factor of 3 on the linear algebra by switching to LU decomposition, without
Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0-521-43108-5)
Copyright (C) 1988-1992 by Cambridge University Press. Programs Copyright (C) 1988-1992 by Numerical Recipes Software.
Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machinereadable
files (including this one) to any server computer, is strictly prohibited. To order Numerical Recipes books or CDROMs, visit website
http://www.nr.com or call 1-800-872-7423 (North America only), or send email to directcustserv@cambridge.org (outside North America).