Perfect graphs constitute a well-studied graph class with a rich
structure, reflected by many characterizations with respect to
Perfect graphs are, for instance, precisely those graphs G
where the stable set polytope STAB(G) coincides
with the fractional stable set polytope QSTAB(G).
For all imperfect graphs G it holds that STAB(G) ⊂ QSTAB(G).
It is, therefore, natural to use the difference between the two polytopes
in order to decide how far an imperfect graph is away from being perfect.
We discuss three different concepts, involving
the facet set of STAB(G),
the disjunctive index of QSTAB(G), and
the dilation ratio of the two polytopes.
Including only certain types of facets for STAB(G),
we obtain graphs that are in some sense close to perfect graphs,
for example minimally imperfect graphs, and
certain other classes of so-called rank-perfect graphs.
The imperfection ratio has been introduced by
Gerke and McDiarmid  as
the dilation ratio of STAB(G) and QSTAB(G),
whereas Aguilera et al.  suggest to take
the disjunctive index of QSTAB(G) as the imperfection index of G.
For both invariants there exist no general upper bounds, but there
are bounds known for the imperfection ratio of several graph
Outgoing from a graph-theoretical interpretation of the imperfection
we prove that there exists no upper bound on the imperfection index
for those graph classes with a known bounded imperfection ratio.
Comparing the two invariants on those classes, it seems that the
imperfection index measures imperfection much more roughly than the
imperfection ratio; we, therefore, discuss possible directions for