Risk management for plant protection products: Higher uncertainties by combining multiple measures?

In the environmental risk assessment of plant protection products, the risk of spray drift losses to the off-field can be reduced by adding drift reduction measures. The question is how the combination of several measures affects the overall risk and, in particular, the associated uncertainty. Various statistical methods have been used to quantify both the overall risk and its uncertainty, in particular the increase in power and the width of a 95% confidence interval where the additional measures are modelled as qualitative factors. It can be concluded that for most of the scenarios considered, power is reduced or only marginally increased. I.e., adding further measures tends to increase uncertainty. This is mainly due to the fact that the power of the 'distance' factor is already very high and it is naturally difficult to increase it further. This relationship is even stronger for the confidence interval model, where it actually increases as additional factors are included in the experimental design with increasing sample size. These trends have also been confirmed empirically using selected experimental data from the SETAC DRAW database, albeit only selectively.