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Individual light curves of active galactic nuclei (AGNs) are nowadayssuccessfully modelled with the damped random walk (DRW) stochastic process,characterized by the power exponential covariance matrix of the signal, withthe power $\beta=1$. By Monte Carlo simulation means, we generate mock AGNlight curves described by non-DRW stochastic processes ($0.5\leq\beta\leq 1.5$and $\beta\neq1$) and show they can be successfully and well-modelled as asingle DRW process, obtaining comparable goodness of fits. A good DRW fit, infact, may not mean that DRW is the true underlying process leading tovariability and it cannot be used as a proof for it. When comparing the input(non-DRW) and measured (DRW) process parameters, the recovered time scale(amplitude) increases (decreases) with the increasing input $\beta$. Inpractice, this means that the recovered DRW parameters may lead to biased (oreven non-existing) correlations of the variability and physical parameters ofAGNs if the true AGN variability is caused by non-DRW stochastic processes. Theproper way of identifying the processes leading to variability aremodel-independent structure functions and/or power spectral densities and thenusing such information on the covariance matrix of the signal in light curvemodelling.

A Degeneracy in DRW Modelling of AGN Light Curves