  We study the NH distribution in a complete sample of 88 AGN selected in the
20-40 keV band from INTEGRAL/IBIS observations. We find that the fraction of
absorbed (NH > 10^{22} cm^2) sources is 43% while the Compton thick AGN
comprise 7% of the sample. While these estimates are fully compatible with
previous soft gamma-ray surveys, they would appear to be in contrast with
results reported by Risaliti et al. (1999) using an optically selected sample.
This apparent difference can be explained as being due to a selection bias
caused by the reduction in high energy flux in Compton thick objects rendering
them invisible at our sensitivity limit. Taking this into account we estimate
that the fraction of highly absorbed sources is actually in close agreement
with the optically selected sample. Furthermore we show that the measured
fraction of absorbed sources in our sample decreases from 80% to about 20-30%
as a function of redshift with all Compton thick AGN having z < 0.015. If we
limit our analysis to this distance and compare only the type 2 objects in our
sample with the Risaliti et al. objects below this redshift value, we find a
perfect match to their NH distribution. We conclude that in the low redshift
bin we are seeing almost the entire AGN population, from unabsorbed to at least
mildly Compton thick objects, while in the total sample we lose the heavily
absorbed 'counterparts' of distant and therefore dim sources with little or no
absorption. Taking therefore this low z bin as the only one able to provide the
'true' distribution of absorption in type 1 and 2 AGN, we estimate the fraction
of Compton thick objects to be >24%.
