We present the final installment of an intensive 13 year study of variations of the optical continuum and broad Hβ emission line in the Seyfert 1 galaxy NGC 5548. The database consists of 1530 optical continuum measurements and 1248 Hβ measurements. The Hβ variations follow the continuum variations closely, with a typical time delay of about 20 days. However, a year-by-year analysis shows that the magnitude of emission-line time delay is correlated with the mean continuum flux. We argue that the data are consistent with the simple model prediction between the size of the broad-line region and the ionizing luminosity, r α L^1/2 _ion. Moreover, the apparently linear nature of the correlation between the Hβ response time and the nonstellar optical continuum F_opt arises as a consequence of the changing shape of the continuum as it varies, specifically F_opt α F ^.56 _UV