  In the Friedmann cosmology the deceleration of the expansion $q$ plays a
fundamental role. We derive the deceleration as a function of redshift $q(z)$
in two scenarios: $\Lambda$CDM model and modified Chaplygin gas ($MCG$) model.
The function for the $MCG$ model is then fitted to the cosmological data in
order to obtain the cosmological parameters that minimize $\chi^2$. We use the
Fisher matrix to construct the covariance matrix of our parameters and
reconstruct the q(z) function. We use Supernovae Ia, WMAP5 and BAO measurements
to obtain the observational constraints. We determined the present acceleration
as $q_0=-0.60 \pm 0.12$ for the $MCG$ model using the Constitution dataset of
SNeIa and BAO, and $q_0=-0.63 \pm 0.17$ for the Union dataset and BAO. The
transition redshift from deceleration to acceleration was found to be around
$0.6$ for both datasets. We have also determined the dark energy parameter for
the $MCG$ model: $\Omega_{X0}=0.834 \pm 0.028$ for the Constitution dataset and
$\Omega_{X0}=0.854 \pm 0.036$ using the Union dataset.
