Included study characteristics

The first of the evidence sources included a review of participant-reported influences on recruitment to trials and included 29 studies reporting data from 847 potential participants. The studies included those who had consented to take part in a trial (n = 10), those who had declined (n = 7), or both (n = 12). The majority of the studies were conducted in the UK (n = 16) or Europe (n = 6) across various clinical areas with the majority in medicine (n = 11) and pharmaceutical interventions being most frequently tested (n = 11). The second review explored participants’ reasons for not completing a trial until the end. It included 11 studies with findings from 168 people. Again, the majority of studies were based in the UK (n = 5), with a clinical focus on mental health (n = 5), and the most frequently tested interventions were psycho-educational and/or cognitive therapy-based tools (n = 4). All studies across both reviews included adult participants.

Patient important carbon relevant influences on recruitment and retention

Additional file 1: Table S1 describes the recruitment themes and specific components within the themes identified and how they map to carbon-relevant factors. From the original 3 themes and 8 subthemes reported in the included review, we identified 27 individual items (e.g. face-to-face communication preferred when giving trial information). Eleven of these individual participant-reported influences on trial recruitment mapped to five of the ten carbon-relevant factors. These five most frequently coded carbon-relevant factors were ‘trial supplies and equipment’ (n = 9), followed by ‘CTU emissions’ (n = 8), and then ‘trial-specific patient assessment’ (n = 7), ‘meetings and travel’ (n = 6) and ‘trial setup’ (n = 5). The remaining five carbon factors (intervention, data collection and exchange, samples, lab, analysis and trial close out) had 0 participant-reported influences coded. All five carbon factors identified as relevant were coded to items within the theme of ‘trial influence on decision to participate’ with no carbon relevant factors identified for ‘personal influence on decision making’ or ‘the impact of potential outcomes to participate’. The theme of ‘trial influence on decision to participate’ included the following, example, individual items and links to carbon factors:

1.1.2. ‘Written communication beneficial to face-to-face trial information’—linked to carbon-relevant factors of production of trial documents, provision/postage of information, CTU emissions, equipment used by CTUs and in some cases attendance at trial-specific patient assessments for consent.

1.2.2. ‘Comprehensive’ and ‘extensive’ briefing of trial participation was preferred to participants—linked to carbon-relevant factors of production of trial documents, CTU emissions, equipment used by CTUs, visits to sites for training and visits of participants for ‘extensive briefing’.

Additional file 2: Table S2 describes the retention themes and specific components within the themes identified and how they map to carbon-relevant factors. Out of the five original themes (and 10 subthemes) reported in the included review, 23 individual items were identified (e.g. perceptions of recovery as a reason to not continue trial medication). There were fewer carbon-relevant factors coded against the participant-reported influences on retention than recruitment, with 4 out of the 10 carbon factors identified as relevant. These were ‘interventions’ (n = 2), ‘CTU emissions’ (n = 1), ‘meetings and travel’ (n = 1) and ‘trial supplies and equipment’ (n = 1). These factors coded to 2 retention themes: ‘the “it” of aspects of the trial with individual preferences of care and support’ which were all linked to the carbon factor of ‘interventions’ and ‘the compatibility of aspects of trial processes with individual capabilities’. The theme of ‘trial influence on decision to participate’ included the following, example, individual items and links to carbon factors:

2.1.2. Interventions being too technical, too physically demanding and too intensive—linked to carbon-relevant factors through utilities required for intervention delivery and activities or resources required relating to intervention delivery

3.1.3. Communication and cultural issues—linked to carbon-relevant factors through assumptions that CTU emissions, equipment and travel will increase with the need for translation or to enhance communication

Figure 1 provides a graph of the carbon-relevant factors by recruitment and retention influence.

Fig. 1

Carbon-relevant factors applicable to participant-reported influences on trial recruitment and retention