In the IPRA model, a method is proposed to identify the major sources of uncertainty in risk assessment. These include the following input variables: food consumption and residue data, extrapolation factors for within and between species vulnerability, and results of toxicological studies. Statistical techniques, such as bootstrapping, are used to estimate the uncertainty. The resulting uncertainty intervals may guide further research, for example where more resources are necessary to generate data. Uncertainty of results of risk assessment has also become an important issue in risk communication.