Time Total Deviation
        modified total variance scaled by tau^2 / 3
        NIST SP 1065 eqn (28) page 26  <--- formula should have tau squared !?!

def ttotdev(data, rate=1.0, data_type="phase", taus=None):
    """ Time Total Deviation
        modified total variance scaled by tau^2 / 3
        NIST SP 1065 eqn (28) page 26  <--- formula should have tau squared !?!
    """

    (taus, mtotdevs, mde, ns) = mtotdev(data, data_type=data_type,
                                        rate=rate, taus=taus)
    td = taus*mtotdevs / np.sqrt(3.0)
    tde = td / np.sqrt(ns)
    return taus, td, tde, ns