  We use the Constitution supernova, the baryon acoustic oscillation, the
cosmic microwave background, and the Hubble parameter data to analyze the
evolution property of dark energy. We obtain different results when we fit
different baryon acoustic oscillation data combined with the Constitution
supernova data to the Chevallier-Polarski-Linder model. We find that the
difference stems from the different values of $\Omega_{m0}$. We also fit the
observational data to the model independent piecewise constant parametrization.
Four redshift bins with boundaries at $z=0.22$, 0.53, 0.85 and 1.8 were chosen
for the piecewise constant parametrization of the equation of state parameter
$w(z)$ of dark energy. We find no significant evidence for evolving $w(z)$.
With the addition of the Hubble parameter, the constraint on the equation of
state parameter at high redshift isimproved by 70%. The marginalization of the
nuisance parameter connected to the supernova distance modulus is discussed.
