  We review some parametric families of 2-qubit states for which concurrence
and maximal singlet fraction (MSF) have different and even opposite behaviour.
For states considered in this work, maximal achievable fidelity (MAF), a
quantity derived by Verstraete and Verschelde in 2003, shows a better agreement
with concurrence and complies with important features required to be considered
a good entanglement measure.
