fit.f

      SUBROUTINE FIT(X,Y,NDATA,SIG,MWT,A,B,SIGA,SIGB,CHI2,Q)
      DIMENSION X(NDATA),Y(NDATA),SIG(NDATA)
      SX=0.
      SY=0.
      ST2=0.
      B=0.
      IF(MWT.NE.0) THEN
        SS=0.
        DO 11 I=1,NDATA
          WT=1./(SIG(I)**2)
          SS=SS+WT
          SX=SX+X(I)*WT
          SY=SY+Y(I)*WT
11      CONTINUE
      ELSE
        DO 12 I=1,NDATA
          SX=SX+X(I)
          SY=SY+Y(I)
12      CONTINUE
        SS=FLOAT(NDATA)
      ENDIF
      SXOSS=SX/SS
      IF(MWT.NE.0) THEN
        DO 13 I=1,NDATA
          T=(X(I)-SXOSS)/SIG(I)
          ST2=ST2+T*T
          B=B+T*Y(I)/SIG(I)
13      CONTINUE
      ELSE
        DO 14 I=1,NDATA
          T=X(I)-SXOSS
          ST2=ST2+T*T
          B=B+T*Y(I)
14      CONTINUE
      ENDIF
      B=B/ST2
      A=(SY-SX*B)/SS
      SIGA=SQRT((1.+SX*SX/(SS*ST2))/SS)
      SIGB=SQRT(1./ST2)
      CHI2=0.
      IF(MWT.EQ.0) THEN
        DO 15 I=1,NDATA
          CHI2=CHI2+(Y(I)-A-B*X(I))**2
15      CONTINUE
        Q=1.
        SIGDAT=SQRT(CHI2/(NDATA-2))
        SIGA=SIGA*SIGDAT
        SIGB=SIGB*SIGDAT
      ELSE
        DO 16 I=1,NDATA
          CHI2=CHI2+((Y(I)-A-B*X(I))/SIG(I))**2
16      CONTINUE
        Q=GAMMQ(0.5*(NDATA-2),0.5*CHI2)
      ENDIF
      RETURN
      END