Om ( z ) is perfectly consistent with ΛCDM for different data sets. For the comparison of different data sets and different parametrizations, we summarize the minimum value of χ 2 in Table I. From Table I, we observe the reliability of parametrizations when using different data sets. Comparing two different parametrizations, we see that both of them can fit well of the data, while the CPL model is a little better than the Wetterich parametrization for the Csta SNIa and the combination of Csta SNIa with BAO data. To summarize, we have examined the influence of the systematics in different data sets in SNIa and BAO on the fitting results of the CPL parametrization. We found that the tension observed in [11] between low z (Csta+BaoR) and the high z (CMB) data is not a general behavior. By using SNIa with other templates and other BAO data sets, the incompatibility of the CPL parametrization will disappear. This result supports the speculation that the systematics in the data sets can affect the fitting results and leads to different evolution of the DE model [11]. However this answer is still not definite. The different evolutions in Om ( z ) 13 IV. CONCLUSION Data CPL ( χ $^{2}$/DOF) Wetterich Csta 462.06/394 466.33/394 Csta+BaoR 462.43/395 467.62/395 Csta+Bao2 462.45/394 467.73/394 Csta+BaoZ 462.10/394 466.60/394 Csta+Bao4 464.11/396 468.57/396 Csta+Bao4+WMAP5 468.73/399 468.69/399 Sdss2 227.55/285 227.09/285 Sdss2+BaoR 229.77/286 229.45/286 Sdss2+Bao2 229.93/285 229.65/285 Sdss2+BaoZ 228.77/285 228.57/285 Sdss2+Bao4 231.15/287 231.02/287 Sdss2+Bao4+WMAP5 231.71/290 231.94/290 TABLE I: The minimum value of χ 2 for different combinations of data sets and models.