Search engines assign scores to each peptide sequence that matches the theoretical spectrum. These peptide spectrum matchings are called PSMs. All peptide candidates are ranked according to their PSM scores and usually the top hit is reported. However not all top scoring peptide identifications are correct. (e.g., if the correct sequence is not present in the DB, the search engine might still assign another sequence to the spectrum which is wrong.)
Distributions of correct and incorrect matches
Search engines assign scores to each peptide sequence that matches the theoretical spectrum. These peptide spectrum matchings are called PSMs. All peptide candidates are ranked according to their PSM scores and usually the top hit is reported. However not all top scoring peptide identifications are correct. (e.g., if the correct sequence is not present in the DB, the search engine might still assign another sequence to the spectrum which is wrong.)