Dinglas 2015.

Study characteristics
Methods	Two RCTs, individuals randomised.
Data	US, secondary care setting. 1) Trial 1 (mail trial): participants who had been enrolled before the introduction of this survey were sequentially randomised. Total n = 332; mean age 48.8 (SD 15.0) years; 52% females; 20% of ethnic minorities (i.e. African American, Asian, American Indian and Alaskan Native), 24% unemployed; 46% retired or disabled. 2) Trial 2 (Phone trial): non‐responders from the prior mail and those excluded from mail trial due to lack of a correct mailing address were eligible. Total n = 171; mean age 46.0 (SD 14.7) years; 51% females; 26% of ethnic minorities (i.e. African American, Asian, American Indian and Alaskan Native), 26% unemployed; 43% retired or disabled.
Comparisons	1) Trial 1 (Mail trial) participants were randomised to mailed letters every two weeks until the survey was completed. Intervention group 1 received a "personal format letter" in which their mailing address and the return address were handwritten, and a traditional stamp was stamped using the envelope. Intervention group 2 received a "business format letter" in which the addresses were typed, and a commercial stamp‐machine affixed the postage. 2) Trial 2 (Phone trial): started 20 days after the end of the mail trial. These telephone calls were made once weekly by the same caller, for up to 4 weeks, until the participant was reached by telephone or the participant called back and completed the survey. Intervention group receive a personalised voice message. Control group received generic voice message.
Outcomes	Participant to complete the insurance survey.
Notes	Retention period: 1) Trial 1 (mail trial): unclear. However, every two weeks until the survey was completed, or the participant was sent a total of 4 mailings. Trial 2 (Phone trial): unclear. However, once weekly by the same caller, for up to 4 weeks
Risk of bias
Item	Authors' judgement	Support for judgement
Allocation concealment?	Unclear	Not discussed in the paper.
Adequate sequence generation?	Yes	In both the mail and the phone trials, randomisation was performed by a statistician using computer‐generated random numbers with an allocation ratio of 1:1.
Blinding of participants and personnel?	Yes	Given the nature of this study design, outcome assessment was not blinded, but participants were blinded. Unclear about the personnel. unblinding not likely to impact objective outcome
Blinding of outcome assessment?	Yes	Given the nature of this study design, outcome assessment was not blinded. However, objective outcome, participants blinded (did not know there was a study) telephone contact was only those participants who received the scripted message, not those who were spoken to, staff have no/very limited plausible additional opportunity to influence postal response rate once questionnaires sent.
Incomplete outcome data addressed? All outcomes	Yes	No concerns raised.
Free of selective outcome reporting?	Yes	No concerns raised.
Other sources of bias	Yes	No further concerns raised.
Overall Risk of Bias	Unclear	Unclear