Preface This volume contains the papers presented at IIR’18, the 9th Italian Informa- tion 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 fo- rum for stimulating and disseminating research in information retrieval, where Italian researchers (especially young ones) and researchers affiliated 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 five relevant topics: – Evaluation of IR systems – Multimedia IR systems – Machine Learning in IR – Efficiency in query processing – Data Mining – Text Classification 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 on- going 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 com- mittee 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 first was given by Aristides Gionis from Aalto University, Finland, on “Combat- ing polarization in social media”. The second talk was by Dietmar Jannach from Alpen-Adria University, Austria. The talk was entitled “Session-based Recom- mendation: 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. 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 Tkalčič (Program co-Chair), Free University of Bozen-Bolzano, Italy General Chair Aris Anagnostopoulos Sapienza University of Rome, Italy Program Chairs Nicola Tonellotto ISTI-CNR, Pisa, Italy Luca Becchetti Sapienza University of Rome, Italy Marko Tkalčič Free University of Bozen-Bolzano, Italy Steering Committee Fabio Crestani Universtiy of Lugano Tommaso Di Noia Politecnico di Bari Raffaele Perego ISTI-CNR Salvatore Orlando Ca’ Foscari University of Venice Giorgio Maria Di Nunzio University of Padua Franco Maria Nardini ISTI-CNR Domenico Dato Tiscali Paolo Boldi University of Milan Fabrizio Sebastiani ISTI-CNR Program Committee Maristella Agosti University of Padua Giambattista Amati Fondazione Ugo Bordoni Aris Anagnostopoulos Sapienza University of Rome Liliana Ardissono University of Torino Maurizio Atzori University of Cagliari Francesco Barile University of Naples Federico II Luca Becchetti Sapienza University of Rome Ludovico Boratto Eurecat Ilaria Bordino UniCredit R&D Claudio Carpineto Fondazione Ugo Bordoni Matteo Catena ISTI-CNR Fabio Crestani University of Lugano Danilo Croce University of Roma Tor Vergata Berardina Nadja De Carolis University of Bari Marco De Gemmis University of Bari Tommaso Di Noia Politecnico di Bari Giorgio Maria Di Nunzio University of Padua Mehdi Elahi Free University of Bozen - Bolzano Andrea Esuli ISTI-CNR Adriano Fazzone Sapienza University of Rome Nicola Ferro University of Padova 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 Raffaele 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 Tkalčič 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