Advanced Learning to Rank using Tagged Query Terms - Experimental

Terrier’s MatchingOp query language was introduced in Terrier 5.0, and permits additional flexibility in defining features for learning to rank. In particular, along with complex query operators such as #syn #1 and #uwN, Terrier’s query language also allows to #tag terms.

In particular, lets consider a query term1 term2. If we use rewrite the query to include dependency query terms using DependenceModelPreProcess, the query will be rewritten to include #1 and #uw8 operators, as follows:

#combine:0=0.85:1=0.15:2=0.05( #combine(term1 term2) #1(term1 term2) #uw8(term1 term2)).

In addition, each of the query terms will be tagged with a String. All terms tagged with firstmatchscore will be scored in the first pass retrieval. Moreover, DependenceModelPreProcess will add the tag sdm to the #1 and #uw8 operators. Therefore the actual query can be represented as:

#combine:0=0.85:1=0.15:2=0.05:tag=firstmatchscore( #combine(term1 term2) #combine:tag=sdm(#1(term1 term2))  #combine:tag=sdm(#uw8(term1 term2)))

These tags make it easy to identify groups of query terms to use as additional features during learning to rank. For instance, consider the following example feature.list:

WMODELt:PL2
WMODELp1:pBiL
WMODELuw8:pBiL
WMODEL$sdm:pBiL2

In constructing features using such a feature list, the following features would be calculated:

  1. WMODELt:PL2 would calculate PL2 only for plain query terms (e.g. term1 and term2);
  2. WMODELp1:pBiL and WMODELuw8:pBiL would calculate the pBiL weighting model for only the #1 and #uw8 query operators, respectively;
  3. WMODEL$sdm:pBiL2 would the pBiL2 weighting model only for the terms tagged with sdm.

We believe that this advanced functionality allows very expressive feature definitions. Note that any query term expressed in a feature will need to be matched in the first retrieval phase. If you do not wish such terms to be scored in the first phase, you are advised to set the weighting model to Null, which will prevent it affecting the retrieved document set.

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