def choose_NT(N_max, iffreq=True):
    """ calculates number of time samples required to constrain N_max modes
        --- equation (21) from Sanders & Binney (2014) """
    if iffreq:
        return max(200, 9 * N_max ** 3 / 4)
    else:
        return max(100, N_max ** 3 / 2)
