Substitution replaces a pairing(s) awarded from a higher bid preference with a pairing(s) that match the same higher bid preference in order to fit pairings from a lower bid on the block. Many pairings may match a specific bid preference, so substitution is used to try different combinations of matching pairings.

For example, if the N-PBS Scheduler is trying to fit a pairing on the block that matches bid preference 3, but the pairing conflicts with a pairing that matches bid preference 1, the N-PBS Scheduler may try to substitute the pairing matching bid preference 1 with another pairing that also matches bid preference 1 but starts on a different day and doesn’t conflict with the pairing that matches bid preference 3. Substitution allows the N-PBS Scheduler to successfully fit pairings that match both bid preferences on the block.