  The analytical study of disordered system is usually difficult due to the
necessity to perform a quenched average over the disorder. Thus, one may resort
to the easier annealed ensemble as an approximation to the quenched system. In
the study of RNA secondary structures, we explicitly quantify the deviation of
this approximation from the quenched ensemble by looking at the correlations
between neighboring bases. This quantified deviation then allows us to propose
a constrained annealed ensemble which predicts physical quantities much closer
to the results of the quenched ensemble without becoming technically
intractable.
