  We study the incidence of magnetism in white dwarfs from three large and
well-observed samples of hot, cool, and nearby white dwarfs in order to test
whether the fraction of magnetic degenerates is biased, and whether it varies
with effective temperature, cooling age, or distance. The magnetic fraction is
considerably higher for the cool sample of Bergeron, Ruiz, and Leggett, and the
Holberg, Oswalt, and Sion sample of local white dwarfs that it is for the
generally-hotter white dwarfs of the Palomar Green Survey. We show that the
mean mass of magnetic white dwarfs in this survey is 0.93 solar masses or more,
so there may be a strong bias against their selection in the magnitude-limited
Palomar Green Survey. We argue that this bias is not as important in the
samples of cool and nearby white dwarfs. However, this bias may not account for
all of the difference in the magnetic fractions of these samples.
  It is not clear that the magnetic white dwarfs in the cool and local samples
are drawn from the same population as the hotter PG stars. In particular, two
or threee of the cool sample are low-mass white dwarfs in unresolved binary
systems. Moreover, there is a suggestion from the local sample that the
fractional incidence may increase with decreasing temperature, luminosity,
and/or cooling age. Overall, the true incidence of magnetism at the level of 2
megagauss or greater is at least 10%, and could be higher. Limited studies
capable of detecting lower field strengths down to 10 kilogauss suggest by
implication that the total fraction may be substantially higher than 10%.
