## 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