  We present the analysis of a deep colour-magnitude diagram (CMD) of NGC 1831,
a rich star cluster in the LMC. The data were obtained with HST/WFPC2 in the
F555W (~V) and F814W (~I) filters, reaching m_555 ~ 25. We discuss and apply a
method of correcting the CMD for sampling incompleteness and field star
contamination. Efficient use of the CMD data was made by means of direct
comparisons of the observed to model CMDs. The model CMDs are built by an
algorithm that generates artificial stars from a single stellar population,
characterized by an age, a metallicity, a distance, a reddening value, a
present day mass function and a fraction of unresolved binaries. Photometric
uncertainties are empirically determined from the data and incorporated into
the models as well. Statistical techniques are presented and applied as an
objective method to assess the compatibility between the model and data CMDs.
By modelling the CMD of the central region in NGC 1831 we infer a metallicity Z
= 0.012, 8.75 < log(tau) < 8.80, 18.54 < (m-M)_0 < 18.68 and 0.00 < E(B-V) <
0.03. For the position dependent PDMF slope (alpha = -dlog(Phi(M))/dlog(M)), we
clearly observe the effect of mass segregation in the system: for projected
distances R < 30 arcsec, alpha ~ 1.7, whereas 2.2 < alpha < 2.5 in the outer
regions of NGC 1831.
