## Features used by Percolator

Following is a list of the features used by the crux percolator command to represent each peptide-spectrum match (PSM).

Number | Name | Type | Description |
---|---|---|---|

1 | `lnrSp` |
Float | The natural logarithm of the rank of the match based on the Sp score |

2 | `deltLCn` |
Float | The difference between this PSM's XCorr and the
XCorr of the last-ranked PSM for this spectrum, divided by
this PSM's XCorr or 1, whichever is larger. |

3 | `deltCn` |
Float | The difference between this PSM's XCorr and the
XCorr of the next-ranked PSM for this spectrum, divided by
this PSM's XCorr or 1, whichever is larger.
Note that this definition differs from that of the
standard delta Cn reported by SEQUEST^{®}. |

4 | `Xcorr / XCorr Score` |
Float | The SEQUEST cross-correlation score |

5 | `Sp / Sp score` |
Float | The preliminary SEQUEST score. |

6 | `IonFrac` |
Float | The fraction of b and y ions theoretical ions matched to the spectrum |

7 | `Mass` |
Float | The observed mass [M+H]^{+} |

8 | `PepLen` |
Integer | The length of the matched peptide, in residues |

9-11 | `Charge[n]` |
Boolean | Is this a 1+/2+/3+ charged spectrum? |

12 | `enzN` |
Boolean | Is the peptide preceded by an enzymatic (tryptic) site? |

13 | `enzC` |
Boolean | Does the peptide have an enzymatic (tryptic) C-terminus? |

14 | `enzInt` |
Integer | Number of missed internal enzymatic (tryptic) sites |

15 | `lnNumSP` |
Float | The natural logarithm of the number of database peptides within the specified precursor range |

16 | `dM` |
Float | The difference between the calculated and observed mass |

17 | `absdM` |
Float | The absolute value of the difference between the calculated and observed mass |