The online version of this article (doi:10.1186/1471-2288-13-136) contains supplementary material, which is available to authorized users.
MRH, RMD and MV have an academic interest in the EMEP-WRF CTM and its development. There are no other conflicts of interest.
BKB contributed to the design of the study, analysed the data, carried out the simulations and took the lead in drafting the paper. BA provided theoretical statistical expertise and contributed to the design and concept of the study. RWA and PW contributed to the design and concept of the study. MRH and RMD assembled the model data and the model-monitor comparison data sets. MV is the main developer of the EMEP-WRF regional chemistry-transport model and produced the model output. All authors contributed to the drafting of the paper, the interpretation of results and read and approved the final manuscript.
Assessing health effects from background exposure to air pollution is often hampered by the sparseness of pollution monitoring networks. However, regional atmospheric chemistry-transport models (CTMs) can provide pollution data with national coverage at fine geographical and temporal resolution. We used statistical simulation to compare the impact on epidemiological time-series analysis of additive measurement error in sparse monitor data as opposed to geographically and temporally complete model data.
Statistical simulations were based on a theoretical area of 4 regions each consisting of twenty-five 5 km × 5 km grid-squares. In the context of a 3-year Poisson regression time-series analysis of the association between mortality and a single pollutant, we compared the error impact of using daily grid-specific model data as opposed to daily regional average monitor data. We investigated how this comparison was affected if we changed the number of grids per region containing a monitor. To inform simulations, estimates (e.g. of pollutant means) were obtained from observed monitor data for 2003–2006 for national network sites across the UK and corresponding model data that were generated by the EMEP-WRF CTM. Average within-site correlations between observed monitor and model data were 0.73 and 0.76 for rural and urban daily maximum 8-hour ozone respectively, and 0.67 and 0.61 for rural and urban loge(daily 1-hour maximum NO2).
When regional averages were based on 5 or 10 monitors per region, health effect estimates exhibited little bias. However, with only 1 monitor per region, the regression coefficient in our time-series analysis was attenuated by an estimated 6% for urban background ozone, 13% for rural ozone, 29% for urban background loge(NO2) and 38% for rural loge(NO2). For grid-specific model data the corresponding figures were 19%, 22%, 54% and 44% respectively, i.e. similar for rural loge(NO2) but more marked for urban loge(NO2).
Even if correlations between model and monitor data appear reasonably strong, additive classical measurement error in model data may lead to appreciable bias in health effect estimates. As process-based air pollution models become more widely used in epidemiological time-series analysis, assessments of error impact that include statistical simulation may be useful.
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Additional file 1: Estimating instrument/location error (example urban ozone).(DOCX 16 KB)12874_2013_1017_MOESM1_ESM.docx
Additional file 2: Predicting bias in the health effect estimate from theory.(DOCX 17 KB)12874_2013_1017_MOESM2_ESM.docx
Additional file 3: Investigating the magnitude and components of measurement error.(DOCX 21 KB)12874_2013_1017_MOESM3_ESM.docx
Authors’ original file for figure 112874_2013_1017_MOESM4_ESM.tif
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- Measurement error in time-series analysis: a simulation study comparing modelled and monitored data
Barbara K Butland
Richard W Atkinson
Mathew R Heal
Ruth M Doherty
- BioMed Central
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