Fig. 1. The number of years required to reach one person year of exposure to tractor driving in relation of annual tractor-driving time.
The regression analysis regarding the risk of low-back and hip symptoms was only performed on the total group. Regarding hip symptoms, the analysis was only performed on the presence or absence of hip symptoms, with no analysis being made of whether these symptoms were in the left, right or both hips. A new variable, ‘‘exposure to tractor driving’’, was created from the product of average annual tractor-driving time in the previous 10 years and the number of years the farmer had regularly driven tractors. These latter two variables were therefore not included in the final analysis. The exposure was scaled into person years, i.e. divided by 365 and 24. This meant that the exposure to tractor driving depended on the number of hours a farmer drove tractors per year. The variable is visualised in Fig. 1. Note that this does not automatically imply that the risk for symptoms followed the exposure. Age, age when they started working within agriculture, exposure to tractor driving, annual time driving tractors, annual time driving tractors within the work operations, tonnes hay handled on the farm, hours working in the cattle shed, hours milking and months working in the forestry per year were entered as continuous variables, the other variables were categorical.