parametrization is in compatible with combinations of data sets at low and high redshifts. The un-evolving Om ( z ) is also allowed even at low redshifts. From Figs. 3a and 4, we see that the ΛCDM model is not allowed at 1 σ level for the combination of BaoR and Csta, Cstb or Cstc, although the cosmological constant is inside the 1 σ contours of w$_{0}$ and w$_{a}$ in Figs. 1b and 1c. This shows the advantage of Om diagnostic because the uncertainty of Ω$_{m}$$_{0}$ is taken accounted for. In the above discussions we have focused on the commonly used functional form for DE, the CPL parametrization, and argued that the systematics in SNIa and BAO data sets really affects the evolution of the DE and the acceleration of the universe. Besides the systematics of data sets, whether the influences on the evolution of the DE and the acceleration are caused by the specific choice of the parametrization of the DE is another interesting question to ask. In [11], the incompatibility of the CPL ansatz fitting to the data simultaneously at low and high redshifts was attributed to the versatility of the CPL parametrization. Choosing another ansatz of DE parametrization, it was found significantly better than that of CPL, that ansatz can provide a good fit to the combination of Csta+BaoR and the CMB shift data. In the rest of this work we are going to further examine the versatility of the CPL parametrization by considering a different ansatz of DE parametrization and confronting it with different combinations of various data sets as used above. What we are going to study is the Wetterich parametrization of the form [24] When z = 0, w ( z ) = w$_{0}$ , and at large redshift z ≫ 1, w ( z ) ≈ 0. The Wetterich parametriza- tion becomes the constant parametrization w ( z ) = w$_{0}$ when w$_{a}$ = 0. In this model, the dimensionless Hubble parameter for a flat universe is To ensure the positivity of DE, we require that w$_{a}$ ≥ 0. For the convenience of numerical calculation, we take the independent parameters as w a 0 = w$_{0}$/w$_{a}$ and w$_{a}$ . The parameters in this model are p = (Ω$_{m}$$_{0}$ , w a $_{0}$, w$_{a}$ ), the number is the same as those in the case of CPL ansatz. The compatibility checks in confronting with different SNIa, BAO data sets together with CMB data are shown in Figs. 2b-2d. In Figs. 2b-2d, we have used the Csta, Cstd and Sdss2 11 E $^{2}$( z ) = Ω$_{m}$$_{0}$(1 + z ) 3 + (1 − Ω$_{m}$$_{0}$)(1 + z ) $^{3}$[1 + w$_{a}$ ln(1 + z )] 3 w 0 /w a . (19) w ( z ) = w$_{0}$ 1 + w$_{a}$ ln(1 + z ) . (18)