  We derive stellar ages, from evolutionary tracks, and metallicities, from
Stromgren photometry, for a sample of 5828 dwarf and sub-dwarf stars from the
Hipparcos Catalogue. This stellar disk sample is used to investigate the
age-metallicity diagram in the solar neighbourhood. Such diagrams are often
used to derive a so called age-metallicity relation. Because of the size of our
sample, we are able to quantify the impact on such diagrams, and derived
relations, due to different selection effects. Some of these effects are of a
more subtle sort, giving rise to erroneous conclusions. In particular we show
that [1] the age-metallicity diagram is well populated at all ages and
especially that old, metal-rich stars do exist, [2] the scatter in metallicity
at any given age is larger than the observational errors, [3] the exclusion of
cooler dwarf stars from an age-metallicity sample preferentially excludes old,
metal-rich stars, depleting the upper right-hand corner of the age-metallicity
diagram, [4] the distance dependence found in the Edvardsson et al. sample by
Garnett & Kobulnicky is an expected artifact due to the construction of the
original sample. We conclude that, although some of it can be attributed to
stellar migration in the galactic disk, a large part of the observed scatter is
intrinsic to the formation processes of stars.
