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### 7.2 Experimental Setup

To determine whether the automatically identified concepts are a useful resource to improve retrieval performance by updating the query model, we compare our approach (WP-SVM) against a query likelihood (QL) baseline and against RM-1 estimated on pseudo relevant documents. In particular, we obtain the set of pseudo relevant documents in three ways:

1.
on the collection (“normal” pseudo relevance feedback—similar to the approach presented in Chapter 4),
2.
on Wikipedia (similar to the approach presented in Chapter 5  as well as so-called “external expansion” [92337]), and
3.
on automatically linked Wikipedia articles (linked using the approach from Chapter 6), as introduced in the previous section.

So, as reference, we use either the collection (RM (C)) or top-ranked Wikipedia articles (RM (WP)) for query modeling. RM (WP) is obtained using a full-text index of Wikipedia, containing all the fields introduced in the previous chapter and including within-Wikipedia anchortexts and titles. For both RM (WP) and RM (C) we use the top 10 retrieved documents and include the 10 terms with the highest probability in $P\left(t|{\stackrel{̂}{\theta }}_{Q}\right)$, similar to the experimental setup used in Chapter 4 (there, on the TREC-PRF-08 collection, RM-1 obtained its highest retrieval performance when 10 terms were used).

To train the SVM model, we split the topic set of each test collection in a training and test set. For TREC Terabyte 2004–2006, we have 149 topics of which 74 are used for training and 75 for testing. For TREC Web 2009 we have 50 topics and use 5-fold cross validation [344]. Similar to the experiments presented in Chapter 4 and described in Section 3.4 (cf. page 100), we perform a line search of the parameter space to determine the optimal value for ${\lambda }_{Q}$.

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