An Initial Investigation of Query Expansion Bias

In this work, the relationship between performance and retrievability bias is explored when various query expansion methods are employed to aide retrieval. Several parameters are altered, independently, to identify those that have an impact on bias. Parameters altered include; Rocchio's beta, length normalisation parameters, the number of terms added and the number of documents those terms are extracted from. A strong correlation between performance and retrievability bias is identified, suggesting that query expansion increases performance by making the system more biased