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C FUNCTION FCHISQ.F
C
C SOURCE
C BEVINGTON, PAGE 194.
C
C PURPOSE
C EVALUATE REDUCED CHI SQUARE FOR FIT OF DATA
C FCHISQ = SUM ((Y-YFIT)**2 / SIGMA**2) / NFREE
C
C USAGE
C RESULT = FCHISQ (Y, SIGMAY, NPTS, NFREE, MODE, YFIT)
C
C DESCRIPTION OF PARAMETERS
C Y - ARRAY OF DATA POINTS
C SIGMAY - ARRAY OF STANDARD DEVIATIONS FOR DATA POINTS
C NPTS - NUMBER OF DATA POINTS
C NFREE - NUMBER OF DEGREES OF FREEDOM
C MODE - DETERMINES METHOD OF WEIGHTING LEAST-SQUARES FIT
C +1 (INSTRUMENTAL) WEIGHT(I) = 1./SIGMAY(I)**2
C 0 (NO WEIGHTING) WEIGHT(I) = 1.
C -1 (STATISTICAL) WEIGHT(I) = 1./Y(I)
C YFIT - ARRAY OF CALCULATED VALUES OF Y
C
C SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
C NONE
C
FUNCTION FCHISQ (Y,SIGMAY,NPTS,NFREE,MODE,YFIT)
DOUBLE PRECISION CHISQ,WEIGHT
DIMENSION Y(1),SIGMAY(1),YFIT(1)
11 CHISQ=0.
12 IF (NFREE) 13,13,20
13 FCHISQ=0.
GOTO 40
C
C ACCUMULATE CHI SQUARE
C
20 DO 30 I=1,NPTS
21 IF (MODE) 22,27,29
22 IF (Y(I)) 25,27,23
23 WEIGHT=1./Y(I)
GOTO 30
25 WEIGHT=1./(-Y(I))
GOTO 30
27 WEIGHT=1.
GOTO 30
29 WEIGHT=1./SIGMAY(I)**2
30 CHISQ=CHISQ+WEIGHT*(Y(I)-YFIT(I))**2
C
C DIVIDE BY NUMBER OF DEGREES OF FREEDOM
C
31 FREE=NFREE
32 FCHISQ=CHISQ/FREE
40 RETURN
END
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