Left column: two examples of simulated AGN light curves for OGLE-III. Both light curves have the mean magnitude I = 18 mag, SF_∞ = 0.20 mag, but the input timescale is τ_inp = 150 d (top) and 25000 d (bottom). The experiment lasted approximately t_exp = 2800 d. Thus, the ratio of the decorrelation timescale to the experiment length is 0.05 and 9.0, respectively. The red line is the best-fit DRW model and the blue lines present 1σ “error snakes”. Right column: model likelihood surface for the two light curves. The black point is the maximum likelihood point, while the red, green, and blue areas represent Δlnℒ = −0.5, −2.0, and −4.5, corresponding to Δχ² = 1, 4, and 9, (or 1, 2, and 3σ), respectively. The open star marks the input parameters. The bias in the measured parameters is obvious in both cases (dots and stars do not match).