Statistical analyses
We described relevant characteristics by randomisation group for each measure. For each device we estimated the reliability using intraclass correlations (or Rho) and within-person standard deviations using a variance-components model [We calculated the differences in measurement between pairs of devices then assessed the mean within-person differences between pairs of devices using paired t-tests. The assumption that the mean differences were normally distributed was checked by plotting histograms, and Bland-Altman plots (the difference between measures versus the average of the measures from the two devices for each individual) were used to assess whether the variation was dependent on the magnitude of the measurements [We also performed a series of sensitivity analyses to test the robustness of the results. We repeated analyses having: (i) excluded measurements where the devices were administered in the incorrect order (n = 2 for blood pressure, n = 5 for grip strength and n = 1 for lung function); (ii) removed extreme outliers identified using scatter plots (n = 1 for blood pressure and n = 2 for grip strength) and; (iii) used alternative outcome definitions commonly used in analyses. For blood pressure, we considered the mean of three readings [Finally, we used multilevel modelling, as an alternative statistical approach, to estimate the differences between devices, using all available readings rather than a summary measure, in order to account for variance between readings. The models treat the repeated readings as Level 1 and the individual as Level 2 to account for non-independence of measurements from the same person. Model 1 included device treated as a fixed effect. Model 2 also included covariates to account for the order in which the devices were administered and the position of the reading in the sequence (1 to 3 for blood pressure, 1 or 2 for the dominant and non-dominant hands for grip strength, and 1 to 5 for lung function). Model 3 was additionally adjusted for age, sex and, for blood pressure only, body mass index.Data cleaning and management were carried out using Excel, IBM-SPSS Version 22 and STATA 14.0 and analyses were conducted using STATA 15.0.