  In this paper, we propose a Q stability parameter that is more realistic than
those commonly used, and is easy to evaluate [see Eq. (19)]. Using our Q_N
parameter, you can take into account several stellar and/or gaseous components
as well as the stabilizing effect of disc thickness, you can predict which
component dominates the local stability level, and you can do all that simply
and accurately. To illustrate the strength of Q_N, we analyse the stability of
a large sample of spirals from The HI Nearby Galaxy Survey (THINGS), treating
stars, HI and H_2 as three distinct components. Our analysis shows that H_2
plays a significant role in disc (in)stability even at distances as large as
half the optical radius. This is an important aspect of the problem, which was
missed by previous (two-component) analyses of THINGS spirals. We also show
that HI plays a negligible role up to the edge of the optical disc; and that
the stability level of THINGS spirals is, on average, remarkably flat and well
above unity.
