Age, period and cohort effects on the incidence of motorcyclist casualties in traffic crashes
- Correspondence to Professor John Langley, Injury Prevention Research Unit, Dept of Preventive and Social Medicine, Dunedin School of Medicine, University of Otago, PO Box 56, Dunedin 9054, New Zealand;
Contributors JL developed the idea for the study and all authors contributed to the design. AS led the analysis. All authors were involved in the interpretation of data. JDL led the drafting of the article, and all authors were involved in revising it and approving the final version to be published. JDL is the guarantor.
- Accepted 22 May 2012
- Published Online First 30 June 2012
Objectives (1) Estimate age, period and cohort effects for motorcyclist traffic casualties 1979–2008 in New Zealand and (2) forecast the incidence of New Zealand motorcycle traffic casualties for the period 2019–2023 assuming future age, cohort and period effects, and compare these with an estimate based on simple linear extrapolation.
Methods Age-period-cohort (APC) modelling was used to estimate the individual effects of age, period and cohort after adjusting for the other two factors. Forecasting was produced for three period-effect scenarios.
Results After adjusting for cohort and period effects, 15–19-year-olds have substantially elevated risk. The period effect reduced in significance over time until the last period, 2004–2008, where the risk was higher than the preceding period. The 10-year cohorts born 1949–1958, 1954–1963, 1959–1968 and 1964–1973, had elevated risk. The forecasting, based on APC modelling, resulted in the lowest estimates of the future incidence being approximately one-third that of the highest estimate (6641).
Conclusion Trends in motorcycle casualties have been influenced by significant independent age, period and cohort effects. These need to be considered in forecasting future casualties. The selection of the period effect has a significant impact on the estimates. Which period-effect scenario readers choose to accept depends on their views about a wide range of factors which might influence motorcycle use and crash risk over time.
- public health
Funding This research was funded by the University of Otago.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement The data used in this study was sourced from the NZ Ministry of Health. Others who wish to source this information should contact the Ministry of Health.
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