  The velocity dispersion of stars in the solar neighbourhood thin disc
increases with time after star formation. Nordstrom et al. (2004) is the most
recent observational attempt to constrain the age-velocity dispersion relation.
They fitted the age-velocity dispersion relations of each Galactic cardinal
direction space velocity component, U (towards the Galactic centre), V (in the
direction of Galactic rotation) and W (towards the North Galactic Pole), with
power laws and interpreted these as evidence for continuous heating of the disc
in all directions throughout its lifetime. We re-visit these relations with
their data and use Famaey et al. (2005) to show that structure in the local
velocity distribution function distorts the in-plane (U and V) velocity
distributions away from Gaussian so that a dispersion is not an adequate
parametrization of their functions. The age-sigma(W) relation can however be
constrained because the sample is well phase-mixed vertically. We do not find
any local signature of the stellar warp in the Galactic disc. Vertical disc
heating does not saturate at an early stage. Our new result is that a power law
is not required by the data: disc heating models that saturate after ~ 4.5 Gyr
are equally consistent with observations.
