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- Liv, Per, 1979-, et al.
(författare)
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Statistical power and measurement requirements in studies comparing observed postures between groups
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Ergonomics studies comparing working postures and movements between independent groups are often based on observations. The present paper derives and exemplifies procedures for estimating the statistical power of such studies, addressing the effect of different strategies for allocating observations within and between observers. In the simple case of one observer rating the postures of all subjects in the study one or multiple times, a simple t-test is appropriate for testing the group difference, while statistical models acknowledging rating differences between observers are needed when multiple observers are involved. In the one-observer case, analytical power calculations are feasible, while a parametric bootstrapping approach is suggested and practiced in the paper for the multiple-observers case. Using empirical data from a previous study of postures among hairdressers observed from video recordings (percentages of time with the right upper arm elevated less than 15° and more than 90°), the study demonstrates that a considerable gain in power can be obtained by having one observer doing multiple repeated observations as compared to rating postures only once. Distributing a certain number of video recordings between multiple observers resulted in a loss of power when a simple t-test was used to test the group difference, but the comparison could be accomplished without loss of power if all observers were involved in rating both of the compared groups and the statistical model used to analyze data acknowledged variability in rating between observers. When different observers assessed the two compared groups, power decreased considerably. Thus, the study gives guidance for efficient design of posture observation studies comparing groups, as well as for appropriate statistical procedures for analyzing the data.
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| 2. |
- Liv, Per, 1979-, et al.
(författare)
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Uncertainties of calibrated exposure estimates, exemplified by working postures assessed by observation and inclinometry
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Objectives: Many occupational exposure variables can be measured using different instruments, of which some can be considered inferior in terms of precision and/or accuracy while others have a superior performance. Thus, working postures can be assessed by observation, which is known to be associated with errors, while direct measurements using inclinometers are assumed to represent a “golden standard”. A possible bias in results obtained by the ”inferior” instrument can be corrected using regression calibration, but the statistical consequences of this procedure are not fully understood. This paper develops procedures for evaluating the precision of an estimate of “true” exposure after calibration, and illustrates them using data from a study of observed upper arm elevation versus corresponding inclinometer measurements.Methods: Three random coefficient models for estimating the relationship between inferior (observations) and superior (inclinometer) measurements were constructed, taking methodological (observer) variability into account to different extents. Expressions for estimating the uncertainty of a calibrated exposure (posture) mean value were derived, which identify the specific contributions from sample uncertainty and uncertainty associated with determining the calibration parameters.Results: In the example of posture observations, calibration introduced an uncertainty that outweighed the size of the observation bias. Thus, this proved to be an example of calibration not always being appropriate, i.e. in case the trade-off between bias correction and increased uncertainty is unfavorable.Conclusions: Calibration of inferior measurements can be a viable tool to correct for bias, but it may add a considerable uncertainty to the eventual mean exposure estimate. Thus, the trade-off between these two calibration effects needs to be considered in each specific case, and further research is needed on the determinants of the trade-off.
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