  Given a non-maximally entangled state, an operationally significant question
is to quantitatively assess as to what extent the state is away from the
maximally entangled state, which is of importance in evaluating the efficacy of
the state for its various uses as a resource. It is this question which is
examined in this paper for two-qubit pure entangled states in terms of
different entanglement measures like Negativity (N), Logarithmic Negativity
(LN), and Entanglement of Formation (EOF). Although these entanglement measures
are defined differently, to what extent they differ in quantitatively
addressing the earlier mentioned question has remained uninvestigated.
Theoretical estimate in this paper shows that an appropriately defined
parameter characterizing the fractional deviation of any given entangled state
from the maximally entangled state in terms of N is quite different from that
computed in terms of EOF with their values differing up to ~ 15 % for states
further away from the maximally entangled state. Similarly, the values of such
fractional deviation parameters estimated using the entanglement measures LN
and EOF, respectively, also strikingly differ among themselves with the maximum
value of this difference being around 23 %. This analysis is complemented by
illustration of these differences in terms of empirical results obtained from a
suitably planned experimental study. Thus, such appreciable amount of
quantitative non-equivalence between the entanglement measures in addressing
the experimentally relevant question considered in the present paper highlights
the requirement of an appropriate quantifier for such intent. We indicate
directions of study that can be explored towards finding such a quantifier.
