Group-based measurement strategies in exposure assessment explored by bootstrapping

Objectives The precision of mean exposure to pushing was examined in 2 occu pational groups using various combinations of the number of workers and mea surements per worker. Methods The frequency and duration of pushing of the 2 occupational groups was assessed using onsite observation. All data were divided into successiv e periods of 30 minutes of observation. The precision of the group mean exp osure to pushing was expressed by 90% confidence intervals obtained by boot strapping. The effect on the confidence interval of varying numbers of work ers and numbers of periods per worker was examined. Results For both occupational groups there was little precision to be gaine d when > 10 workers were observed. Within the maximum number of workers use d in the bootstrap simulations, it appeared that, beyond 10 workers, the co nfidence intervals decreased by <5% for every worker that was added, when e ach worker was observed at least 8 periods of 30 minutes. If workers were o bserved exactly 4 periods of 30 minutes per worker, an additional 4 workers were required to compensate for the loss of precision. An unbalanced strat egy with approximately 8 periods of 30 minutes per worker hardly decreased the precision of the group mean, however. Conclusions The precision of the group-based mean exposure to pushing is in fluenced by the number of workers observed and by the number of repeated me asurements per worker. In the planning of measurement strategies, it is adv isable to account for possible sources of variance in advance and to assess the exposure variability.