  Context: The damped random walk (DRW) stochastic process is nowadays
frequently used to model aperiodic light curves of AGNs. A number of
correlations between the DRW model parameters, the signal decorrelation
timescale and amplitude, and the physical AGN parameters such as the black hole
mass or luminosity have been reported. Aims: We are interested in whether it is
plausible to correctly measure the DRW parameters from a typical ground-based
survey, in particular how accurate the recovered DRW parameters are compared to
the input ones. Methods: By means of Monte Carlo simulations of AGN light
curves, we study the impact of the light curve length, the source magnitude,
cadence, and additional light on the DRW model parameters. Results: The most
significant finding is that currently existing surveys are going to return
unconstrained DRW decorrelation timescales, because typical rest-frame data do
not probe long enough timescales or the white noise part of PSD for DRW. The
experiment length must be at least ten times longer than the true DRW
timescale, being presumably in the vicinity of one year, meaning a minimum
10-years-long AGN light curves (rest-frame). The DRW timescales for
sufficiently long light curves are typically weakly biased, and the exact bias
depends on the fitting method and used priors. The DRW amplitude is mostly
affected by the photometric noise (so the source magnitude or the
signal-to-noise ratio), cadence, and the AGN host light. Conclusions: Because
the DRW parameters appear to be incorrectly determined from typically existing
data, the reported correlations of the DRW variability and physical AGN
parameters from other works seem unlikely to be correct. In particular, the
anti-correlation of the DRW timescale with redshift is a manifestation of the
survey length being too short. Application of DRW to modeling typical AGN
optical light curves is questioned.
