PRO eval_chi_sqrd_nu, npts, npar, values, varis, predicts, $
    chi_sqrd_nu, q
; Computes the reduced chi square and the probability that
; it exceeds chance agreement.
;
; Author:  Robert Kessel
;          Electro-Optics Technology Section
;          Code 8123
;          Naval Research Laboratory
;          Washington, D.C.  20375-5354
; email:   kessel@ncst.nrl.navy.mil
;
; This code was written for Bill Palya's and Don Walter's
; use at JSU.  However, anyone else who finds it useful
; should feel free to use it, rewrite it, or whatever else,
; save charging any money, goods or services from any other
; person for the code or use thereof.
; 8/8/00

nu = npts - npar

; compute reduced chi square with trap on zero variance
chi_sqrd_nu = 0.
FOR i=0,npts-1 DO BEGIN
   IF (varis[i] GT 0.) THEN BEGIN
     resid_sqrd = (values[i] - predicts[i])^2
     chi_sqrd_nu = chi_sqrd_nu + resid_sqrd/varis[i]
   ENDIF ELSE BEGIN
     PRINT, 'At data point ', i
     PRINT, '    '
     STOP, 'Failed greater than zero variance in eval_chi_sqrd_nu'
   ENDELSE
ENDFOR
chi_sqrd_nu = chi_sqrd_nu/nu

q = q_chi_sqrd_nu(nu, chi_sqrd_nu)

END