{#subsec:data-sources}
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[**Twitter**]{} (<https://twitter.com/>) is one of the largest microblogging platforms used by hundreds of millions every month. To collect Twitter messages (“tweets”) we use an archive representing 10% of the entire public stream, known as the “Decahose.”

[**Reddit**]{} (<https://reddit.com/>) is a large social news aggregation platform used by millions every month. Users submit “posts” and “comments” that gain or lose visibility according to up- and down-votes. We collect posts through Reddit’s Search API[^3] (comments are not searchable via this API), retaining all comments to posts matching our queries.

[**News.**]{} Finally, we collect news articles from GDELT (Global Data on Events, Location, and Tone, <http://gdeltproject.org/>), the largest online catalog of global news events. We use these data as exogenous variables when modeling social media time series before and after a given event.

Query Construction {#subsec:query-construction}
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We collected data using keyword queries, a sampling method applicable to both Twitter’s and Reddit’s APIs. As our goal was to create a high-recall collection, our sampling procedure consists in formulating an initial query (bootstrapping), followed by an expansion of that query. This method is known to improve the coverage of social media data [@olteanu2014crisislex; @davidson2017automated].

[**Query Bootstrapping.**]{}\[concept:query-bootstrapping\] We bootstrapped our query selection with 