  We have summarized all Delta a measurements for galactic field stars (1474
objects) from the literature published over more than two decades. These
measurements were, for the first time, compiled and homogeneously analyzed. The
Delta a intermediate band photometric system samples the depth of the 520nm
flux depression by comparing the flux at the center with the adjacent regions
with bandwidths of 11 to 23nm. Because it was slightly modified over the last
three decades, we checked for systematic trends for the different measurements
but found no correlations whatsoever. The Delta a photometric system is most
suitable to detecting magnetic chemically peculiar (CP) stars with high
efficiency, but is also capable of detecting a small percentage of non-magnetic
CP objects. Furthermore, the groups of (metal-weak) lambda Bootis, as well as
classical Be/shell stars, can be successfully investigated. In addition, we
also analyzed the behaviour of supergiants (luminosity class I and II). On the
basis of apparent normal type objects, the correlation of the three sigma
significance limit and the percentage of positive detection for all groups was
derived. We compared the capability of the Delta a photometric system with the
Delta (V1 - G) and Z indices of the Geneva 7-color system to detect peculiar
objects. Both photometric systems show the same efficiency for the detection of
CP and lambda Bootis stars, while the indices in the Geneva system are even
more efficient at detecting Be/shell objects. On the basis of this statistical
analysis it is possible to derive the incidence of CP stars in galactic open
cluster and extragalactic systems including the former unknown bias of
undetected objects.
