def VarCircle(XY, Par):  # must have at least 4 sets of xy points or else division by zero occurs
    """
    computing the sample variance of distances from data points (XY) to the circle Par = [a b R]
    """
    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