  By interpreting the well-known, qualitative criteria for the existence of
quantum error correction (QEC) codes by Knill and Laflamme from a quantitative
perspective, we propose a figure of merit for assessing a QEC scheme based on
the average fidelity between codewords. This enables us to quantify the
performance of a protocol as a whole, including errors beyond the correctable
set. Various examples are calculated for photonic qubit codes dealing with the
experimentally relevant case of photon loss, demonstrating the advantages of
the new measure. In the context of continuous-variable QEC, our
codeword-overlap measure can be used to reproduce, in a different way with no
need for calculating entanglement measures, a previous result concerning the
impossibility of improving transmission of Gaussian states through Gaussian
channels via Gaussian operations alone.
