code: def VarCircle(XY, Par):  # must have at least 4 sets of xy points or else division by zero occurs
      
      if type(XY) != numpy.ndarray:
          XY = numpy.array(XY)
      n = len(XY)
      if n < 4:
          raise Warning("Circle cannot be calculated with less than 4 data points.  Please include more data")
      Dx = XY[:,0] - Par[0]
      Dy = XY[:,1] - Par[1]
      D = numpy.sqrt(Dx * Dx + Dy * Dy) - Par[2]
      result = old_div(numpy.dot(D, D),(n-3))
      return result
docs: computing the sample variance of distances from data points (XY) to the circle Par = [a b R]