Peterson, B., Berlind, P., Bertram, R., Bochkarev, N., Bond, D., Brotherton, M., …Younger, P. (1994). Steps toward determination of the size and structure of the broad-line region in active nuclei. 7: Variability of the optical spectrum of NGC 5548 over years. Astrophysical Journal, 425, 622-634. https://doi.org/10.1086/174009