SUBROUTINE chstwo(bins1,bins2,nbins,knstrn,df,chsq,prob)
      INTEGER knstrn,nbins
      REAL chsq,df,prob,bins1(nbins),bins2(nbins)
CU    USES gammq
      INTEGER j
      REAL gammq
      df=nbins-knstrn
      chsq=0.
      do 11 j=1,nbins
        if(bins1(j).eq.0..and.bins2(j).eq.0.)then
          df=df-1.
        else
          chsq=chsq+(bins1(j)-bins2(j))**2/(bins1(j)+bins2(j))
        endif
11    continue
      prob=gammq(0.5*df,0.5*chsq)
      return
      END
C  (C) Copr. 1986-92 Numerical Recipes Software .