def non_gaussian(onset, frame):
    """
    Calculate the Non-Gaussian parameter :
    ..math:
      \alpha_2 (t) = \frac{3}{5}\frac{\langle r_i^4(t)\rangle}{\langle r_i^2(t)\rangle^2} - 1
    """
    r_2 = ((frame - onset)**2).sum(axis=-1)
    return 3 / 5 * (r_2**2).mean() / r_2.mean()**2 - 1