How Do Astronomers Share Data? Reliability and Persistence of Datasets Linked in AAS Publications and a Qualitative Study of Data Practices among US Astronomers Alberto Pepe 1,2 *, Alyssa Goodman 1,2 , August Muench 1 , Merce Crosas 2 , Christopher Erdmann 1 1 Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States of America, 2 Institute for Quantitative Social Science, Harvard University, Cambridge, Massachusetts, United States of America Abstract We analyze data sharing practices of astronomers over the past fifteen years. An analysis of URL links embedded in papers published by the American Astronomical Society reveals that the total number of links included in the literature rose dramatically from 1997 until 2005, when it leveled off at around 1500 per year. The analysis also shows that the availability of linked material decays with time: in 2011, 44% of links published a decade earlier, in 2001, were broken. A rough analysis of link types reveals that links to data hosted on astronomers’ personal websites become unreachable much faster than links to datasets on curated institutional sites. To gauge astronomers’ current data sharing practices and preferences further, we performed in-depth interviews with 12 scientists and online surveys with 173 scientists, all at a large astrophysical research institute in the United States: the Harvard-Smithsonian Center for Astrophysics, in Cambridge, MA. Both the in-depth interviews and the online survey indicate that, in principle, there is no philosophical objection to data-sharing among astronomers at this institution. Key reasons that more data are not presently shared more efficiently in astronomy include: the difficulty of sharing large data sets; over reliance on non-robust, non-reproducible mechanisms for sharing data (e.g. emailing it); unfamiliarity with options that make data-sharing easier (faster) and/or more robust; and, lastly, a sense that other researchers would not want the data to be shared. We conclude with a short discussion of a new effort to implement an easy-to-use, robust, system for data sharing in astronomy, at theastrodata.org, and we analyze the uptake of that system to-date. Citation: Pepe A, Goodman A, Muench A, Crosas M, Erdmann C (2014) How Do Astronomers Share Data? Reliability and Persistence of Datasets Linked in AAS Publications and a Qualitative Study of Data Practices among US Astronomers. PLoS ONE 9(8): e104798. doi:10.1371/journal.pone.0104798 Editor: Aaron Alain-Jon Golden, Albert Einstein College of Medicine, United States of America Received November 20, 2013; Accepted July 18, 2014; Published August 28, 2014 Copyright: ß 2014 Pepe et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This material is based upon work supported by the National Aeronautics and Space Administration under Grant/Contract/Agreement No. NNX12AE11G issued through the Astrophysical Data Analysis Program (ADAP). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * Email: alberto.pepe@gmail.com Introduction No, I don’t have a website where I store these data. Most of it is in various stages of mess. —An Astronomer Astronomical observations can generate very large volumes of data, and observations taken at a particular time are by definition irreplaceable and unrepeatable. As such, making astronomical data publicly available in a structured, intelligible format is of fundamental importance to enable scientific transparency and long term data curation and preservation, facilitating data re-use [1]. To date, some of the most systemically planned data sharing in astronomical research has focused on the preservation and dissemination of observations created in so-called ‘‘sky surveys.’’ The purpose of these surveys is to collect and measure data from extended regions of the Sky, in a systematic and controlled fashion. Modern optical sky surveys, such as the Sloan Digital Sky Survey (SDSS), the 2-Micron All-Sky Survey (2MASS), and the future Large Synoptic Survey Telescope (LSST) generate massive databases, ranging in size from hundreds of terabytes to hundreds of petabytes [2]. Surveys that rely on spectrally-resolved observa- tions, often made with radio-wavelength interferometers, generate ‘‘3D Data Cubes’’ rather than ‘‘2D images,’’ and they are already so large that it is not possible to keep all the raw data after analysis is complete. Despite their sheer volume, the data collected in the context of large surveys represent only a portion of all the data generated in Astronomy. Most discoveries rely upon smaller studies, and/or are based on heavily-processed subsets of many surveys. In any field of scientific endeavor, many different levels of data exist [3]: from ‘‘raw’’ data to ‘‘processed’’ data, from ‘‘calibration’’ data to ‘‘published’’ data. If we imagine all data in Astronomy to be a pyramid, primary data from large sky surveys occupies the bottom half of the pyramid. But, as we just mentioned, these primary data are used by astronomers all over the world to produce more specific studies, where astronomers analyze and process primary data in many ways producing derived data. PLOS ONE | www.plosone.org 1 August 2014 | Volume 9 | Issue 8 | e104798