  The method to estimate the predictability of human mobility was proposed in
[C. Song \emph{et al.}, Science {\bf 327}, 1018 (2010)], which is extensively
followed in exploring the predictability of disparate time series. However, the
ambiguous description in the original paper leads to some misunderstandings,
including the inconsistent logarithm bases in the entropy estimator and the
entropy-predictability-conversion equation, as well as the details in the
calculation of the Lempel-Ziv estimator, which further results in remarkably
overestimated predictability. This paper demonstrates the degree of
overestimation by four different types of theoretically generated time series
and an empirical data set, and shows the intrinsic deviation of the Lempel-Ziv
estimator for highly random time series. This work provides a clear picture on
this issue and thus helps researchers in correctly estimating the
predictability of time series.
