  We examine in some detail the influence of the systematics in different data
sets including type Ia supernova sample, baryon acoustic oscillation data and
the cosmic microwave background information on the fitting results of the
Chevallier-Polarski-Linder parametrization. We find that the systematics in the
data sets does influence the fitting results and leads to different evolutional
behavior of dark energy. To check the versatility of Chevallier-Polarski-Linder
parametrization, we also perform the analysis on the Wetterich parametrization
of dark energy. The results show that both the parametrization of dark energy
and the systematics in data sets influence the evolutional behavior of dark
energy.
