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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
include <mach.h>
include "gsurfitdef.h"
define COV Memr[P2P($1)] # element of COV
# GSERRORS -- Procedure to calculate the reduced chi-squared of the fit
# and the standard deviations of the coefficients. First the variance
# and the reduced chi-squared of the fit are estimated. If these two
# quantities are identical the variance is used to scale the errors
# in the coefficients. The errors in the coefficients are proportional
# to the inverse diagonal elements of MATRIX.
procedure gserrors (sf, z, w, zfit, chisqr, errors)
pointer sf # curve descriptor
real z[ARB] # data points
real w[ARB] # array of weights
real zfit[ARB] # fitted data points
real chisqr # reduced chi-squared of fit
real errors[ARB] # errors in coefficients
int i, nfree
real variance, chisq, hold
pointer sp, covptr
begin
# allocate space for covariance vector
call smark (sp)
call salloc (covptr, GS_NCOEFF(sf), TY_REAL)
# estimate the variance and chi-squared of the fit
variance = 0.
chisq = 0.
do i = 1, GS_NPTS(sf) {
hold = (z[i] - zfit[i]) ** 2
variance = variance + hold
chisq = chisq + hold * w[i]
}
# calculate the reduced chi-squared
nfree = GS_NPTS(sf) - GS_NCOEFF(sf)
if (nfree > 0)
chisqr = chisq / nfree
else
chisqr = 0.
# if the variance equals the reduced chi_squared as in the
# case of uniform weights then scale the errors in the coefficients
# by the variance not the reduced chi-squared
if (abs (chisq - variance) <= DELTA)
if (nfree > 0)
variance = chisq / nfree
else
variance = 0.
else
variance = 1.
# calculate the errors in the coefficients
# the inverse of MATRIX is calculated column by column
# the error of the j-th coefficient is the j-th element of the
# j-th column of the inverse matrix
do i = 1, GS_NCOEFF(sf) {
call aclrr (COV(covptr), GS_NCOEFF(sf))
COV(covptr+i-1) = 1.
call rgschoslv (CHOFAC(GS_CHOFAC(sf)), GS_NCOEFF(sf),
GS_NCOEFF(sf), COV(covptr), COV(covptr))
if (COV(covptr+i-1) >= 0.)
errors[i] = sqrt (COV(covptr+i-1) * variance)
else
errors[i] = 0.
}
call sfree (sp)
end
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