  We revisit a model-independent estimator for cosmic acceleration based on
type Ia supernovae distance measurements. This approach does not rely on any
specific theory for gravity, energy content or parameterization for the scale
factor or deceleration parameter and is based on falsifying the {\it null
hypothesis} that the Universe never expanded in an accelerated way. By
generating mock catalogues of known cosmologies we test the robustness of this
estimator establishing its limits of applicability. We detail the pros and cons
of such approach. For example, we find that there are specific counterexamples
in which the estimator wrongly provides evidence against acceleration in
accelerating cosmologies. The dependence of the estimator on the $H_0$ value is
also discussed. Finally, we update the evidence for acceleration using the
recent UNION2.1 and JLA samples. Contrary to recent claims, available data
strongly favors an accelerated expansion of the Universe in complete agreement
with the standard $\Lambda$CDM model.
