The effects of infrequent but difficult input conditions on N-version programming

The sofware reliability of a system based on N-version programming is dominated by how well frequent but difficult input conditions are handled. In traditional single version programming, these inputs are responsible for only a small percentage of software failures. As versions are added using N-version programming techniques, failures due to common and simple input conditions are quickly reduced. But failures due to infrequent but difficult conditions are only slightly reduced. For a system with a large number of versions, the vast majority of failures are due to infrequent but difficult input conditions. This complicates the prediction of sofware reliability and effects how an N-version programming system should be implemented.