Continuum and emission-line variability of the Seyfert galaxy Arakelian 120 - analysis of a large database

New spectrophotometric observations are combined with observations from earlier papers to construct a homogeneous data base of 107 optical continuum measurements and 75 H-beta flux measurements of the Seyfert 1 galaxy Akn 120. Statistical analysis of these data reveals that: (1) the individual measurements are apparently accurate to better than about 10 percent; (2) there is no reliable evidence for real continuum or emission-line variations on timescales shorter than about 20 days; and (3) there is substantial evidence for real variations in both the line and the continuum on timescales in the range of about 20-40 days. Indeed, large-amplitude (nearly a factor of 2) emission-line changes have been detected on several occasions over intervals shorter than two months. The correlation between the continuum and emission-line changes is consistent with the earlier claim that the broad-line region is quite small in this galaxy. 24 refs