Group-based measurement strategies in exposure assessment explored by bootstrapping

Objectives. The precision of mean exposure to pushing was examined in 2 occupational groups using various combinations of the number of workers and measurements per worker. Methods. The frequency and duration of pushing of the 2 occupational groups was assessed using onsite observation. All data were divided into successive periods of 30 minutes of observation. The precision of the group mean exposure to pushing was expressed by 90% confidence intervals obtained by bootstrapping. The effect on the confidence interval of varying numbers of workers and numbers of periods per worker was examined. Results. For both occupational groups there was little precision to be gained when>10 workers were observed. Within the maximum number of workers used in the bootstrap simulations, it appeared that, beyond 10 workers, the confidence intervals decreased by