  Some fluids are known to behave anomalously. The so-called structural anomaly
which means that the fluid becomes less structures under isothermal compression
is among the most frequently discussed ones. Several methods for quantifying
the degree of structural order are described in the literature and are used for
calculating the regions of structural anomalies. It is implied that all of the
structural order determinations yield qualitatively identical results. However,
no explicit comparison was made. This paper presents such a comparison for the
first time. the results of some definitions are shown to contradict the
intuitive notion of a fluid. On the basis of this comparison we show that the
structural anomaly can be most reliably determined from the behavior of the
excess entropy.
