A 2x2 randomised factorial SWAT of the use of a pen and small, financial incentive to improve recruitment in a randomised controlled trial of yoga for older adults with multimorbidity

Introduction

Efficient recruitment to randomised controlled trials (RCTs) is important to achieve the target sample size and statistical power within the planned budget and time frame. Incentives, monetary or otherwise, are sometimes used to increase trial recruitment.¹ Financial incentives have been found to increase recruitment by 4% (95% CI -1–8%) in a meta-analysis.¹ However, most of the included studies used an incentive of £100, which is larger than publicly funded trials can usually afford. There remains, therefore, uncertainty as to whether financial incentives should be used and, if so, what amount.

Offering a potential participant a gift such as a pen may make them more likely to take up the invitation to enrol in a trial. It is also possible that the convenience of having a pen to hand upon receipt of the invitation may help facilitate a swifter response. However, a previous SWAT conducted by the York Trials Unit evaluated the use of pens as an incentive for recruitment into the OTIS trial of older adults and showed no difference in proportion of participants randomised (pen 4.5%; no pen 4.3%, odds ratio (OR) 1.04, 95% CI 0.65–1.67, p = 0.86), or screened (pen 14.2%, no pen 11.7%, OR 1.25, 95% CI 0.94–1.67, p = 0.12), or in time to return screening form (hazard ratio (HR) 1.23, 95% CI 0.94–1.60, p = 0.13).² To our knowledge, this is the only previous RCT to evaluate pens to increase trial recruitment, so more evidence was needed. We conducted a methodological ‘study within a trial’ (SWAT) to evaluate the effects of including a small, unconditional financial incentive and/or a pen in the postal recruitment pack on the proportion of participants randomised into the host trial. We hypothesised that receipt of a pen or financial incentive would improve the response to the trial invite, encourage a faster response and ultimately result in more participants being randomised into the host trial. We expected an additive effect of receiving both incentives, and that they would not interact.

Methods

Interventions

Financial incentive

A £5 (GBP cash) note was enclosed within the recruitment pack for the host trial. The amount of £5 was chosen as we wanted to include a note, rather than coins, as they are lighter (and so did not add any additional postage costs). £5 is the smallest denomination of GBP note; this was deemed sufficient as a thank you to participants for considering taking part in GYY, and including anything larger, e.g. £10, was less likely to be cost-effective.

Pen

A retractable ballpoint, black ink pen, branded with the GYY trial logo, was enclosed within the recruitment pack for the host trial (Figure 1).

Figure 1.

GYY SWAT Pen.

Financial incentive + pen

Both a pen and the cash note were enclosed within the recruitment pack for the host trial.

Control

The host trial recruitment pack was sent without the inclusion of a pen or cash.

Results

In total, 852 allocations were generated but, due to one of the participating GP practices having a shorter mailing list than anticipated, only 818 (96.0%) were used (Table 1; Figure 2).

Figure 2.

Participant flow diagram.

Twenty-six (3.2%) SWAT participants were randomised into the host trial (Table 2). There was no evidence that the proportion of participants randomised was increased by including a pen (pen: 15/409, 3.7%; no pen: 11/409, 2.7%; OR 1.38, 95% CI 0.63–3.04, p = 0.43) or £5 (£5: 14/409, 3.4%; no £5: 12/409, 2.9%; OR 1.18, 95% CI 0.54–2.57, p = 0.69) in the recruitment packs. The interaction between the interventions was investigated as a secondary analysis and was not found to be statistically significant (interaction coefficient 0.98, 95% CI 0.20–4.79, p = 0.98).

There was no evidence that including a pen increased the proportion of participants returning a screening form (pen: 66/409, 16.1%; no pen: 61/409, 14.9%; OR 1.10; 95% CI 0.75–1.61, p = 0.61), but there was strong evidence for including £5 (£5: 77/409, 18.8%; no £5: 50/409, 12.2%; OR 1.67; 95% CI 1.13–2.45, p = 0.01). The interaction between the interventions was investigated as a secondary analysis and was not found to be statistically significant (interaction coefficient 1.66, 95% CI 0.76–3.60, p = 0.20).

There was no evidence of a difference in time to return a screening form associated with inclusion of a pen (HR 1.09; 95% CI 0.77–1.55, p = 0.61), but including £5 decreased the time to return a screening form (HR 1.56; 95% CI 1.09–2.22, p=0.02). See Kaplan–Meier plots (Figure 3). The Grambsch and Therneau test did not indicate deviation from the proportional hazards assumption. ³ The interaction between the interventions was investigated as a secondary analysis and was not found to be statistically significant (interaction coefficient 1.56, 95% CI 0.76–3.19, p = 0.22).

Figure 3.

Kaplan–Meier curves for time to return screening form.

The additional cost of including a pen in the postal mailout was £0.32; the inclusion of £5 additionally cost only the value of the note itself. Given the 1% increase in participants randomised when sent a pen, 100 (1/0.01) pens would need to be sent to recruit one additional participant at a cost of £32 (100×£0.32). Given the 0.5% increase in participants randomised when sent £5, we would need to send 200 participants £5, at a cost of £1000, to recruit one extra participant ((1/0.005)*5).

Discussion

There was no evidence of a difference in the proportion of participants randomised into the host trial between those sent a pen or £5 and those who did not receive the respective incentive. The proportions of participants randomised in the ‘no intervention’ arms (2.7% and 2.9%) were similar to the 3% assumed in the sample size calculation but the observed group differences were smaller than the 4% difference we were powered for; therefore, this SWAT was underpowered to detect the differences observed.

There was little or no evidence that sending a pen increased the proportion of participants returning a screening form or decreased time to return the form.

A small, monetary incentive did not result in more participants being randomised into the host trial but was effective at prompting return of the screening form, and of a swifter return. Since sending a financial incentive (even a small one) is, by its very nature, relatively costly, we do not recommend this intervention for use to increase recruitment in older adults with multimorbidity. The pen was cheaper but provided little evidence of benefit. If the observed effect of a 1% difference was true then we would need sufficient SWATs to provide an overall sample size of around 11,000 participants to confirm this. In a meta-analysis with the OTIS SWAT, the pooled OR associated with receipt of a pen was 1.12 (95% CI 0.75–1.67, p = 0.58) (Figure 4). Because the extra cost of recruiting an additional participant is relatively small, more SWATs are required to assess whether this difference is a true effect, since sending pens could be a cost-effective intervention for recruitment.

Figure 4.

Meta-analysis of inclusion of a pen in postal recruitment packs on randomisation into host trial.

Although no statistically significant interactions between the pen and £5 were observed, as was expected, this cannot be ruled out as the sample size of this trial was likely insufficient to be powered to detect an interaction.

For both the pen and cash, the same wording on the participant information sheet was used, which stated that the pen and/or £5 were complimentary and sent as a thank you for people considering taking part in the host trial. Some anecdotal evidence from the GYY trial’s process evaluation suggested that participants felt it unnecessary to receive £5 with their recruitment pack as they would willingly have joined the trial without this purely to help themselves, others and the research. In addition, this may have caused potential confusion if participants discussed receiving £5 during their yoga sessions as to why some received it and some did not. Such sentiments were not expressed in relation to being sent a pen, potentially suggesting that people view non-monetary incentives differently (more like a gift) than monetary incentives.

We were unable to obtain views on receiving the incentives from people sent a recruitment pack but whom either did not return it, were ineligible or declined participation in the GYY host trial. Future SWATs may want to consider ways to obtain qualitative accounts from such participants, as this would add further useful context in which to interpret the findings.

In conclusion, we did not find evidence that the inclusion of a pen and/or £5 was particularly effective or represented good value for money for improving recruitment into a trial of Gentle Years Yoga for older adults with multimorbidity.

Data availability