Theoretical predictions for the ensemble quasar structure function are tested using multi-epoch observations of Stripe 82 collected by the Sloan Digital Sky Survey .
We reanalyze the entire available volume of the g -band imaging data using difference image photometry and build high quality light curves for 7562 spectroscopically confirmed quasars .
Our structure function includes \sim 4.8 \times 10 ^ { 6 } pairs of measurements and covers a wide range of time lags between 3 days and 6.9 years in the quasar rest frame .
A broken power-law fit to this data shows the presence of two slopes \alpha _ { 1 } = 0.33 and \alpha _ { 2 } = 0.79 with the break at \sim 42 days .
The structure function compiled using only flux increases is slightly lower than that for variations of the opposite sign , revealing a slight asymmetry between the leading and trailing edge of a typical flare .
The reality of these features is confirmed with monte-carlo simulations .
We give simple interpretation of the results in the frames of existing theoretical models .