  We show that the Cepheid PL relation is affected by a Malmquist type bias,
the so-called population incompleteness bias. Its calculated slope appears
shallower than the true one because of the cutoff in apparent magnitude
resulting from the instrumental limiting magnitude. Furthermore, the use of the
PL relation, even with the correct slope, leads to underestimation of distances
which is not negligible. We confirm this finding by studying simulated PL
relations, and we show that this bias may be as large as 0.2 or 0.3 magnitudes
on distance modulus. We also test the efficiency of a cutoff in $\log P$ and
show that it is a good way to minimize this bias. However, a correction of this
is difficult as long as the completeness of the sample is not perfectly well
established.
