  We investigate the relationship between the halo mass, M_200, and
concentration, c, for a sample of 26 group- and cluster-scale strong
gravitational lenses. In contrast with previous results, we find that these
systems are only ~ 0.1 dex more over-concentrated than similar-mass halos from
dark matter simulations; the concentration of a halo with M_200 = 10^14 M_sun
is log c = 0.78\pm0.05, while simulations of halos with this mass at similar
redshifts (z ~ 0.4) predict log c ~ 0.56 - 0.71. We also find that we are
unable to make informative inference on the slope of the M_200-c relation in
spite of our large sample size; we note that the steep slopes found in previous
studies tend to follow the slope in the covariance between M_200 and c,
indicating that these results may be measuring the scatter in the data rather
than the intrinsic signal. Furthermore, we conclude that our inability to
constrain the M_200-c slope is due to a limited range of halo masses, as
determined by explicitly modelling our halo mass distribution, and we suggest
that other studies may be producing biased results by using an incorrect
distribution for their halo masses.
