Evaluation of IR systems 0 Multimedia IR systems 0 Machine Learning in IR 0 E ciency in query processing 0 Data Mining 0 Text Classi cation 0 Workshop Organisers: Aris Anagnostopoulos (General Chair), Sapienza University of Rome, Italy Nicola Tonellotto (Program co-Chair), ISTI-CNR, Pisa, Italy Luca Becchetti (Program co-Chair), Sapienza University of Rome, Italy Marko Tkalcic (Program co-Chair), Free University of Bozen-Bolzano , Italy

This volume contains the papers presented at IIR'18, the 9th Italian Information Retrieval Workshop held on May 28-30, 2018 in Rome, Italy. The purpose of the Italian Information Retrieval (IIR) workshop series is to provide a forum for stimulating and disseminating research in information retrieval, where Italian researchers (especially young ones) and researchers a liated with Italian institutions can network and discuss their research results in an informal way. Previously IIR workshops took place in Lugano (2017), Venice (2016), Cagliari (2015), Rome (2014), Pisa (2013), Bari (2012), Milan (2011) and Padua (2010). The contributions to IIR 2018 mainly address ve relevant topics: In this edition, a complete track has been devoted to Recommender Systems as an emerging IR-related topic in the Italian community. We received 35 submissions, both full and short original papers presenting new research results as well as extended abstracts containing descriptions of ongoing projects or summarizing already published results. Each submission was reviewed by at least two program committee members. The reviewers looked at originality, technical depth, style of presentation, and impact. Finally, the committee decided to accept 30 papers for presentation at the workshop, 18 of which appear in these proceedings. The program also included three invited talks. The rst was given by Aristides Gionis from Aalto University, Finland, on \Combating polarization in social media". The second talk was by Dietmar Jannach from Alpen-Adria University, Austria. The talk was entitled \Session-based Recommendation: Challenges and Recent Advances". The last talk was given by Nicola Ferro from University of Padua, Italy and it focused on \The Long Road Ahead from Explanation to Prediction of IR System Performance". All invited talks were highly praised by participants.

 Aris Anagnostopoulos Sapienza University of Rome, Italy Program Chairs Nicola Tonellotto Luca Becchetti Marko Tkalcic Steering Committee Fabio Crestani Tommaso Di Noia Ra aele Perego Salvatore Orlando Giorgio Maria Di Nunzio Franco Maria Nardini Domenico Dato Paolo Boldi Fabrizio Sebastiani Program Committee ISTI-CNR, Pisa, Italy Sapienza University of Rome, Italy Free University of Bozen-Bolzano, Italy Universtiy of Lugano Politecnico di Bari ISTI-CNR Ca' Foscari University of Venice University of Padua ISTI-CNR Tiscali University of Milan ISTI-CNR Fabio Gasparetti Rome Tre University Cristina Gena University of Torino Pasquale Lops University of Bari

Claudio Lucchese Ca' Foscari University of Venice

 Ida Mele ISTI-CNR Massimo Melucci University of Padua Stefano Mizzaro University of Udine Alejandro Moreo Fernndez ISTI-CNR Maurizio Morisio Politecnico di Torino Alessandro Moschitti Qatar Computing Research Institute Cristina Ioana Muntean ISTI-CNR Cataldo Musto University of Bari Franco Maria Nardini ISTI-CNR Fedelucio Narducci University of Bari Vito Claudio Ostuni Pandora Umberto Panniello Politecnico di Bari Ra aele Perego ISTI-CNR Massimo Quadrana Politecnico di Milano Francesco Ricci Free University of Bozen-Bolzano Silvia Rossi University of Naples Federico II Giuseppe Sansonetti Roma Tre University

Fabrizio Sebastiani Qatar Computing Research Institute

 Giovanni Semeraro University of Bari Fabrizio Silvestri Facebook

Panagiotis Symeonidis Free University of Bozen-Bolzano Marko Tkalcic Free University of Bozen-Bolzano

Gabriele Tolomei University of Padua Paolo Tomeo Politecnico di Bari Nicola Tonellotto ISTI-CNR Salvatore Trani ISTI-CNR Rossano Venturini University of Pisa

Guido Zuccon Queensland University of Technology