Optional argument.
Either an instance of MSnIDFilter class or a "character".
level
Level at which the filter will be evaluated. Possible values are
"PSM", "peptide" and "accession". Multiple are OK. Default is all of them.
Value
Returns a matrix with with column names "fdr" and "n".
Column "n" contains the number of features (spectra,
peptides or proteins/accessions) passing the filter.
Column "fdr" is the false discovery rate (i.e. identification
confidence) for the corresponding features.
Row names correspond to the provided levels.