  Determination of mass composition of high energy cosmic rays is one of the
greatest challenge in modern astrophysics. All of previous methods for finding
the mass composition of primary cosmic rays in a surface array at least require
two independent measurements (e.g. muon and electron components) of extensive
air showers (EAS). Here a new statistical parameter is introduced which can be
used to determine the mass composition of vertical downward cosmic rays in a
simple surface array. The most interesting thing about the new parameter is
that it does not need two independent measurements and can be used in a simple
surface array which does not have muon detectors.
