## Appendix A
### Qualitative Assessment of Buses about Perception of Credibility  
Aiming to pair the quantitative analysis we performed in Section 4 with qualitative feedback, we deployed an ex post survey, asking 10 media experts to share their experience and their beliefs about how they perceive news articles online, depending on the auxiliary information they can read. The methodology we used was the Delphi method [58]: it consists of several rounds of survey (typically two or three) where domain experts are called to answer questions anonymously, preventing any of the experts from influencing their peers due to their authority. Between the two rounds, a facilitator collected and summarised participants' answers, which were then reported to them. In the subsequent round, the experts could be asked to revise their answers or to answer more in-depth questions. The process was terminated after a given number of rounds or after convergence is reached. In its original formulation, a Delphi survey was a tool especially suitable for collective forecasting of a given phenomenon based on expert opinions. For our study, we asked 10 social media and Web experts to answer a few questions designed after the main findings of our analysis. In spite of the spirit of a Delphi survey, the goal of this analysis was to collect thoughtful answers from a panel of experts, rather than to reach convergence. Briefly, we asked how the credibility of news can be affected by contextual hints, such as the source of an article, the informative richness of the article's preview on social networks, and what other users thought of the article; we also asked how often users rely on external fact-checking, and what age users are more likely to fact-check news. Round 1 questions were designed as open questions, which let participants point out their opinion with no constraints; Round 2 questions were designed as multiple choice questions, specifically targeted at the main topics and themes that emerged after Round 1. In the following, we report Round 1's questions:  
* Q1: **What are, based on your experience and competencies, the main indicators of the credibility of a news article on social networks? ** Participants spontaneously converged on the source of the article (9 out of 10), followed at a distance by the sources cited by the article, the coverage of the news across diverse sources, and who the social network users are that broke the news. * Q2: **In your experience, what makes you think a news piece could be false? ** Participants pointed out mainly the poor quality of the news (8 out of 10), but also an unreliable publisher (5 out of 10) and a lack of support in other news outlets (3 out of 10). * Q3: **Based on your experience, how do you rate the quality and quantity of information reported in the news previews on social networks? ** Participants found the quality of news previews to be poor (7 out of 10), with a slight tendency to clickbait, regardless of the news publisher. Additionally, the quantity of information was pointed out as insufficient (4 out of 10). * Q4: **Does there exist, according to you, a relationship between news credibility and the response of users on social networks? If you think it exists, could you explain what you think it is? ** A total of 6 out of 10 participants believed that bold claims, especially from false news publishers, result in higher arousal among readers, which can also foster news diffusion. However, 4 out of 10 participants did not acknowledge any direct relationship between credibility and public response. * Q5: **How do you make sure a piece of news is credible when you are in doubt, in your experience as a user? ** Participants unanimously indicated a parallel search on other news sources. Additionally, 4 out of 10 participants mentioned the need for checking on reliable sources, such as debunking websites.