In collaboration with nine UK press offices, we ran a randomised controlled trial in which the ‘participants’ were press releases (N = 312) distributed to international media outlets over a 20-month period from September 2016 to May 2017. To operationalise evidence strength, we concentrated on the basic distinction between correlational and experimental types of evidence, a keystone for assessing the ability to support causal conclusions .
The collaborating press offices sent their biomedical and health-related press releases to us just prior to release. We randomly allocated each press release to receive one, both or neither of two interventions. The first intervention was causal claim alignment. We made suggestions to align the headline and prominent claims with the evidence, such that direct causal claims were only made for experimental evidence, while correlational data carried cautious claims, using words such as might and may. The second intervention was a causality statement/caveat. We inserted an explicit statement about whether the evidence could support a causal conclusion (e.g. this was an observational study, which does not allow us to conclude that drinking wine caused the increased cancer risk).
The press office was then free to accept, edit or reject the proposals (sometimes in consultation with academics according to their normal procedures) and issued the release as normal. We searched for arising news (print, online and broadcast; total N = 2257), and its content was double-coded by two researchers blind to condition and press release content. The protocol was pre-registered (https://doi.org/10.1186/ISRCTN10492618, 20/08/2015) and approved by the Research Ethics Committee at the School of Psychology, Cardiff University. We do not name press offices to avoid identifying individuals. All data are available online at https://osf.io/apc6d/
Participants: press releases
The ‘participants’ in the trial were press releases. For inclusion criteria, see Fig. 1.
We estimated we would achieve 300–500 press releases based on 100% coverage of eligible press releases from participating offices. In practice, some offices released fewer relevant press releases than expected and some eligible press releases were not sent to us for a variety of reasons (Fig. 1; 261 of 499 eligible press releases were sent; see reasons beyond the exclusion criteria of joint release and author consent). We therefore extended the trial duration and introduced a stopping rule of 75 press releases per bin (prior to exclusion of study designs not classifiable as experimental or correlational). Since we used pure randomisation, some bins were larger than others (Additional file 1: Table S2) and the total was 312 following study-design exclusion. Note that the power calculations in the protocol are only indications, since actual power depended on the clustering structure in the GEE analyses.
Randomisation and blinding
Randomisation was by independent random number generation for each press release received (and therefore allowed unequal cell sizes by chance) and occurred prior to any assessment of content (and therefore before exclusion of simulations and mixed-methods reviews which reduced some cells below 75; Table 1). We did not communicate the condition to the press office. There were three researchers coordinating the trial (RCA, AC and LB). For each batch of press releases, RCA or AC coordinated randomisation and interventions, while the other two would remain blind for double-coding the outcomes.
Causal claim alignment
The main causal claims in the headline and body of the press release were altered to align with the evidence underlying those claims. If claims were already aligned with the evidence, these were not modified. Based on previous results  showing which causal phrases readers distinguish or treat equivalently, all claims for observational evidence were modified to use hedged/cautious or associative language (may, could, might; e.g. ‘drinking wine may increase cancer risk’; associated, linked; e.g. ‘drinking wine is associated with increased cancer risk’) unless such language was already used. Claims for experimental evidence were modified to (or left as) direct causal statements (e.g. ‘drinking wine increases cancer risk’) or can cause statements (‘drinking wine can increase cancer risk’). In the registered protocol, we referred to alignment as accuracy (see Additional file 1: Figure S2).
Unless it already existed, a statement was inserted into the press release body to convey the design of the study and the strength of causal conclusions that could be justified from this design. For example, ‘this was an observational study, which does not allow us to conclude that drinking wine caused the increased cancer risk’ or ‘this study was a randomised controlled trial, which is one of the best ways for determining whether an intervention has a causal effect’ (in the registered protocol, we labelled this intervention study design statement; see Additional file 1: Figure S2). These statements were inserted at the earliest point where they fitted with the press release content. The majority were inserted into text, not into quotes, because feedback from press officers indicated that it was normally not pragmatic to get author approval for new quotes before release.
Causal claim alignment + causality statement
In this condition we suggested changes according to both A and B above, unless they were already present.
The control condition was a suggested synonym change for a word that was not relevant to the main causal claims or study design (e.g. ‘beverage’ changed to ‘drink’).
From each pre-intervention press release, a list of search terms was generated to search for print, online and broadcast news coverage from a pre-defined list of top-tier national and international news outlets (see Additional file 1: Figure S3). Searches were conducted using Nexis, Google and TV Eyes. News coverage was sourced for 1 week prior to the press release date (to cover date differences due to time zones and any breaches of embargo) and for 28 days following the release. Two researchers blind to condition and final press release content coded the news using a standard protocol abbreviated from Sumner et al.  to extract the content outcomes listed below. All discrepancies in coding were resolved so that the final concordance was 100%. See open data for the full coding sheet.
Causal headline and claim alignment: We coded whether the news headline and news main claims were direct causal, can cause or hedged causal/associative. Alignment was defined relative to the study design of the peer-reviewed journal article. Following Adams et al. (2017), we grouped direct cause and can cause together as strong claims appropriate for experimental evidence, and we refer to hedged cause/associative statements as cautious claims appropriate for correlational evidence . We coded and analysed headlines and main claims separately as they are normally written by different people (sub-editor and journalist); headlines are most prominent but the writers are one step further removed from the press release. We operationalised main claims as those made in the first two sentences beyond the headline (excluding context sentences not about the new study). We excluded news headlines or claims that were not causal/associative or made a claim of no cause (‘wine does not raise cancer risk’). We also excluded news claims that were about entirely different variables than the press release.