  The measurement of the average depth of the shower maximum is the most
commonly used observable for the possible inference of the primary cosmic-ray
mass composition. Currently, different experimental Collaborations process and
present their data not in the same way, leading to problems in the
comparability and interpretation of the results. Whereas <Xmax> is expected to
be proportional to <ln A> in ideal conditions, we demonstrate that the finite
field-of-view of fluorescence telescopes plus the attenuation in the atmosphere
can introduce a non-linearity into this relation, which is specific for each
particular detector setup.
