  Event-by-event fluctuations of the chemical composition of the hadronic final
state of relativistic heavy-ion collisions carry valuable information on the
properties of strongly interacting matter produced in the collisions. However,
in experiments incomplete particle identification distorts the observed
fluctuation signals. The effect is quantitatively studied and a new technique
for measuring chemical fluctuations, the identity method, is proposed. The
method fully eliminates the effect of incomplete particle identification. The
application of the identity method to experimental data is explained.
