  We present a Bayesian method to determine simultaneously the age,
metallicity, distance modulus, and interstellar reddening by dust of any
resolved stellar population, by comparing the observed and synthetic color
magnitude diagrams on a star by star basis, with no need to bin the data into a
carefully selected magnitude grid. We test the method with mock stellar
populations, and show that it works correctly even for scarce stellar
populations with only one or two hundred stars above the main sequence turn
off. If the population is the result of two star formation bursts, we can infer
the contribution of each event to the total stellar population. The code works
automatically and has already been used to study massive amounts of Magellanic
clouds photometric data. In this paper we analyze in detail three Large
Magellanic Cloud star clusters and 6 Ultra Faint Dwarf Galaxies. For these
galaxies we recover physical parameters in agreement with those quoted in the
literature, age $\sim13.7$ Gyr and a very low metallicity $log\,Z\sim-4$.
Searching for multiple populations in these galaxies, we find, at a very low
significance level, signs of a double stellar population for Ursa Major I: a
dominant old population and a younger one which contributes $\sim25$% of the
stars, in agreement with independent results from other authors.
