Online supplementary table S1 describes how each study defined its study population as ‘low income’. Twenty-three studies reported having measured participants’ check FAQ income as part of the study. Varying thresholds and income groupings were applied, but most commonly, incomes below US$15–US$20 000 (approximately £8840–11 800) per year were considered ‘low’ and most studies reported that the majority of participants were in this category.

Of the remaining 12 studies, 8 recruited participants from financial support programmes which required beneficiaries’ earnings to be equivalent or near to official US poverty levels (which vary over time and depending on the individual’s household size), 2 reported that the majority of participants held a manual, low wage occupation and the final 2 studies reported that participants’ neighbourhoods had a high proportion of residents living in poverty.

Following initial identification, participants were recruited through face-to-face contact, via letter, telephone, via media advertisement or most commonly a mixture of methods. Face-to-face opportunities described were door-to-door neighbourhood recruitment, organisation of a community health fair, invitation at medical or social services appointments, or through presentations at schools or other community groups. Telephone calls were usually a follow-up method of contact. Media advertisements included posters in community

venues, newspaper, radio and television advertisements. In the majority of cases, it was the study investigators who initiated these recruitment activities. Timeframe of recruitment varied from 1 day to over 2 years. Techniques used to engage low-income groups in participating were poorly specified: those most commonly reported were offers of material incentives (eg, vouchers for signing up), prompts and cues (eg, a fridge magnet with the study telephone number) or social support Batimastat to facilitate participation (eg, advising about crèche facilities). Study design and participant characteristics The characteristics of the 35 included studies are summarised in online supplementary table S1. The majority (k=30) were conducted in the USA; the remaining studies were from the UK (k=3), Australia (k=1) and Chile (k=1). Twenty-eight studies were RCTs; seven were cRCTs. Studies took place in community (k=22), healthcare (k=12) or workplace (k=1) settings.