title: Statistical performance of observational work sampling for assessment of categorical exposure variables: a simulation approach illustrated using PATH data.
abstract: OBJECTIVE
  Observational work sampling is often used in occupational studies to assess categorical biomechanical exposures and occurrence of specific work tasks. The statistical performance of data obtained by work sampling is, however, not well understood, impeding informed measurement strategy design. The purpose of this study was to develop a procedure for assessing the statistical properties of work sampling strategies evaluating categorical exposure variables and to illustrate the usefulness of this procedure to examine bias and precision of exposure estimates from samples of different sizes.
  
  
  METHODS
  From a parent data set of observations on 10 construction workers performing a single operation, the probabilities were determined for each worker of performing four component tasks and working in four mutually exclusive trunk posture categories (neutral, mild flexion, severe flexion, twisted). Using these probabilities, 5000 simulated data sets were created via probability-based resampling for each of six sampling strategies, ranging from 300 to 4500 observations. For each strategy, mean exposure and exposure variability metrics were calculated at both the operation level and task level and for each metric, bias and precision were assessed across the 5000 simulations.
  
  
  RESULTS
  Estimates of exposure variability were substantially more uncertain at all sample sizes than estimates of mean exposures and task proportions. Estimates at small sample sizes were also biased. With only 600 samples, proportions of the different tasks and of working with a neutral trunk posture (the most common) were within 10% of the true target value in at least 80% of all the simulated data sets; rarer exposures required at least 1500 samples. For most task-level mean exposure variables and for all operation-level and task-level estimates of exposure variability, performance was low, even with 4500 samples. In general, the precision of mean exposure estimates did not depend on the exposure variability between workers.
  
  
  CONCLUSIONS
  The suggested probability-based simulation approach proved to be versatile and generally suitable for assessing bias and precision of data collection strategies using work sampling to estimate categorical data. The approach can be used in both real and hypothetical scenarios, in ergonomics, as well as in other areas of occupational epidemiology and intervention research. The reported statistical properties associated with sample size are likely widely relevant to studies using work sampling to assess categorical variables.
authors.affiliation: 1. Department of Occupational and Public Health Sciences, Centre for Musculoskeletal Research, University of Gävle, Kungsbäcksvägen, SE- 80176 Gävle, Sweden;
authors.forename: Svend Erik
authors.initials: SE
authors.lastname: Mathiassen
authors.forename: Jennie A
authors.initials: JA
authors.lastname: Jackson
authors.forename: Laura
authors.initials: L
authors.lastname: Punnett
mesh_terms: D001696:Biomechanical Phenomena; D001699:Biometry; D003198:Computer Simulation; D060389:Construction Industry; D003625:Data Collection / Q000379:methods*; D006801:Humans; D016273:Occupational Exposure / Q000706:statistics & numerical data*; D011187:Posture; D011336:Probability; D017132:Workplace
keywords: epidemiology; ergonomics; exposure assessment methodology; precision; statistical efficiency; working postures