def effective_area(true_energy, reco_energy, simu_area):
    """
    Compute the effective area from a list of simulated energy and reconstructed energy
    Parameters
    ----------
    true_energy: 1d numpy array
    reco_energy: 1d numpy array
    simu_area: float - area on which events are simulated
    Returns
    -------
    float = effective area
    """
    return simu_area * len(reco_energy) / len(true_energy)