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. 2014 Mar 10;16(3):e78. doi: 10.2196/jmir.3010

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©Paolo Bajardi, Alessandro Vespignani, Sebastian Funk, Ken TD Eames, W John Edmunds, Clément Turbelin, Marion Debin, Vittoria Colizza, Ronald Smallenburg, Carl E Koppeschaar, Ana O Franco, Vitor Faustino, Annasara Carnahan, Moa Rehn, Daniela Paolotti. Originally published in the Journal of Medical Internet Research (http://www.jmir.org), 10.03.2014.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work, first published in the Journal of Medical Internet Research, is properly cited. The complete bibliographic information, a link to the original publication on http://www.jmir.org/, as well as this copyright and license information must be included.

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Panel A: Illustration of study definitions for enrollment and follow-up participation. Definition for enrollment: first symptoms questionnaire (SQ) at least 60 days before end of monitoring season and at least one more SQ within 15 days from first SQ; Definition for follow-up participation: at least two SQs during time window of 30 to 60 days after first SQ. Panel B: The numbers of Influenzanet users and enrolled participants, obtained applying the enrollment definition given in Panel A, are shown. The final sample on which the multilevel regression was performed, after the complete-subject exclusion of individuals with missing records, was of 22,968 enrolled individuals who provided information for all the variables. Among these, after applying the definition for follow-up participation given in Panel A, 16,161 were respondent participants and 6,807 had a lower participation in the follow-up.