1 INTRODUCTION
1.1 Motivation
1.2 The Landscape of Explainable Information Retrieval
1.3 Methodology and Scope
2 NOTIONS AND CATEGORIZATION
2.1 Notions in Explainable Information Retrieval
2.2 Post-hoc Interpretability
2.3 Interpretability by Design
2.4 Grounding to Information Retrieval Principles
2.5 Evaluation of Explanations
3 FEATURE ATTRIBUTION
3.1 Model-agnostic Feature Attribution
3.2 Model-introspective Feature Attribution
3.3 Evaluating Feature Attributions
4 FREE-TEXT EXPLANATIONS
4.1 Explaining Query Intent
4.2 Explaining Document Relevance
4.3 Evaluation of Free-text Explanations
5 ADVERSARIAL EXAMPLES
5.1 Adversarial Examples in Ranking
5.2 Universal Adversarial Triggers
6 AXIOMATIC ANALYSIS OF TEXT RANKING MODELS
6.1 Interpretable Axiomatic Rankers
6.2 Axioms for Model Diagnostics
6.3 Axioms for Regularizing Neural Rankers
6.4 Evaluation
7 PROBING AND PARAMETRIC ANALYSIS OF TEXT RANKING MODELS
7.1 The Probing Methodology
7.2 Probing Ranking Models
7.3 Probing other Information Retrieval Models
7.4 Evaluation
8 EXPLAINABLE-BY-ARCHITECTURE MODELS
8.1 Explainable Text Rankers
8.2 Explainable Learning-to-Rank
8.3 Evaluation
9 RATIONALE-BASED METHODS
9.1 Pipeline Approaches
9.2 End-to-End Approaches
9.3 Evaluation
10 LIMITATIONS AND OPEN QUESTIONS
10.1 Limitations
10.2 Open Questions
11 CONCLUSION