3 Protocol for the In Balance randomized controlled trial | 51 Economic evaluation For the economic evaluation, missing cost and effect data will be imputed using multiple imputation according to the algorithm developed by van Buuren et al. (178). Linear regression analyses will be used to estimate cost and effect differences between the intervention group and the control group while adjusting for confounders if necessary. Bias-corrected accelerated bootstrapping with 5000 replications will be used to estimate statistical uncertainty surrounding the cost and effect differences. Incremental cost-effectiveness ratios (ICERs) will be calculated by dividing the difference in costs between the groups by the difference in effects. Bootstrapped cost-effect pairs will be plotted on cost-effectiveness planes to show the uncertainty surrounding the ICER. Cost-effectiveness acceptability curves will be estimated to show the probability that the In Balance intervention is cost-effective compared to control for different willingness-to-pay values (i.e. the amount of money that society is willing to pay per additional unit of effect). Adverse events Adverse events are defined as any undesirable experience occurring to a participant during the study, whether or not considered related to the In Balance intervention or the trial procedure. All adverse events reported spontaneously by the participant or observed by the investigator or staff will be recorded. Occurrence of adverse events will be assessed during the monthly telephone calls and for the intervention group during the In Balance intervention by the therapist. Trial status Enrolment into the study will start on September 1, 2021. We plan to complete the recruitment and data collection at baseline by September 2022, with a 12 months follow-up period on falls incidence and incidence of fall-related injuries until September 2023. This study has been registered with the Netherlands Trial Register on 13 February 2021 (NL9248). DISCUSSION This paper describes the design of a (cost-)effectiveness study comparing an implemented group-intervention to prevent falls, In Balance, with general written physical activity recommendations. Exercise programmes have been proven to be effective in preventing falls. In particular, interventions containing challenging balance and functional elements result in the most beneficial outcomes (46, 179, 180). The In Balance intervention includes both balance and physical exercises, and is already proven to be effective in reducing the number of falls among a frail, residential population (55). Since the target population and duration of the In Balance intervention programme intervention have been adjusted after broader
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