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### 4.5 Summary and Conclusions

Relevance assessments by a user are an important and valuable source of information for retrieval. In a language modeling setting, various methods have been proposed to estimate query models from them. In this chapter we have considered several core relevance feedback approaches for query modeling. Some of these models base their estimations on the set of feedback documents, whereas others base them on each individual document. We have presented two novel query modeling methods that incorporate both sources of evidence in a principled manner. One of them, MLgen, employs the probability that the set of relevant documents generated the individual documents. The other, NLLR, leverages the distance between each relevant document and the set of relevant documents to inform the query model estimates and, as such, it is more general than methods proposed before. Our chief aim in this chapter was to present, analyze, and evaluate these two novel models. Our second aim was to present a thorough evaluation of various core relevance feedback models for language modeling under the same experimental conditions on a variety of test collections.

From performing a large number of experiments on four test collections using the same experimental conditions, we have arrived at a number of conclusions. First, under pseudo relevance feedback, there is a large variance in the resulting retrieval performance for different amounts of pseudo relevant documents, most notably on large, noisy collections, such as .GOV2 and ClueWeb09. The same effect, although less pronounced, is observed for the number of terms that are included in the query models. It is typical to compare retrieval performance of relevance feedback models using a fixed setting of documents and terms. Given the results presented in this chapter, however, this strategy is not recommended since the relative performance might change considerably for small changes in the values for these parameters. We have also concluded that the test collection itself is of influence on the relative performance of the models; there is no single model that outperforms the others on all test collections. Furthermore, the optimal values for ${\lambda }_{Q}$, $|\stackrel{̂}{R}|$, and $|{\mathsc{V}}_{Q}|$ also varies between test collections. Moreover, we have found that the optimal values for these parameters vary when one optimizes either for early precision or for MAP. On the TREC Robust 2004 test collection, a collection commonly used when evaluating pseudo relevance feedback models, we find that the models under investigation behave very differently than on the more realistically sized web collections. Furthermore, on TREC Robust 2004 most models behave very similarly when varying the parameter settings we have investigated in this chapter. We found that RM-1 has the most robust performance. That is, although this model does not obtain the highest performance, it is only moderately sensitive to the various parameter settings and the terms it includes in the query models are changed only slightly when these values change. This stability is caused by the way RM-1 gathers evidence. First, it aggregates relevance feedback information per query term, after which it looks at the documents. Hence, the query terms function as a kind of “filter,” primarily causing the query terms to be reweighted. The novel models we presented earlier in this chapter, MLgen and NLLR, perform quite differently on pseudo relevance feedback. NLLR only slightly outperforms the baseline on TREC-PRF-08 and is substantially worse on the other test collections. MLgen, on the other hand, obtains close to the best performance on both TREC-PRF-08 and TREC-WEB-09.

As to the observations made when using explicit relevance feedback, here we found that the variance with respect to the number of feedback documents is much less pronounced. We also find that explicit relevance feedback does not unanimously help; some topics are hurt, whilst others are helped. This is a common finding when using pseudo relevance feedback, but the experimental results presented in this chapter have shown that this is also the case for explicit relevance feedback. However, when averaged over a number of topics, we find that all relevance feedback models improve over a QL baseline when using explicit relevance feedback information. The NLLR and PRM models obtain the highest performance using explicit relevance feedback, although MLgen and RM-0 also fare well.

Let’s turn to the research question formulated earlier in this chapter.

RQ 1.
What are effective ways of using relevance feedback information for query modeling to improve retrieval performance?

Using extensive experiments on three test collections (for pseudo relevance feedback) and one test collection (for explicit relevance feedback), we found that using relevance feedback models yields substantial, and in most cases significant, improvements over the baseline. In particular, we found that the PRM model obtains the highest scores on most test collections. Furthermore, we found that RM-1 yields the most robust performance (i.e., being the least sensitive to various parameter settings) under pseudo relevance feedback on two test collections. Finally, our proposed NLLR model is particularly suited for use in combination with explicit relevance feedback.

This general research question gave rise to the following subquestions.

RQ 1a.
Can we develop a relevance feedback model that uses evidence from both the individual feedback documents and the set of feedback documents as a whole? How does this model relate to other query modeling approaches using relevance feedback? Is there any difference when using explicit relevance feedback instead of pseudo relevance feedback?

We have presented two novel models that aim to make use of both of these sources of information and have compared to a number of other, established relevance feedback models for query modeling. In theoretical terms, we have shown that these related methods can be considered special cases of NLLR which, under explicit relevance feedback, is able to reap the benefits of all the methods it subsumes. Using pseudo relevant feedback documents, the performance of our models leaves room for improvement. Under explicit relevance feedback, however, we have shown that NLLR is particularly suitable for use in conjunction with this type of feedback. The other proposed model, MLgen, behaves similar to the related models, both under explicit and pseudo relevance feedback.

RQ 1b.
How do the models perform on different test collections? How robust are our two novel models on the various parameters query modeling offers and what behavior can we observe for the related models?

We have found that there exists a large variance in the performance of all evaluated models on different test collections. Furthermore, the number of documents used for estimation and the number of terms included in the query models exhibit a considerable influence on the retrieval performance. Properly optimizing these parameters (either for recall- or precision-oriented measures) yields substantial and mostly significant improvements on the measure optimizing for.

In the next chapter, we introduce and evaluate a query modeling approach for annotated documents, i.e., documents annotated using concepts. This novel method builds upon the intuitions behind the relevance modeling approach, as well as MBF and PRM. Using our two-step method, we find that using information from the annotations helps to significantly improve end-to-end retrieval performance. After we have presented a method for linking queries to concepts in Chapter 6, we turn to using these concepts for query modeling (again using relevance feedback techniques) in Chapter 7.

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