Uncertainty is a growing concern in hydrology. Several types of uncertainty, data, parameter and model uncertainty and the consequent output uncertainty, have been studied previously. Despite these uncertainties, hydrological predictions and their uncertainties are assumed to be objective and to be independent of the author/modeller. In recent years, the variance of modelling decisions on model output (the influence of a modeller) has been acknowledged more often. No extensive quantification of this variance has been computed yet. A preliminary study to this variance was performed. However, this study was limited to one catchment and four modelling decisions. In this PhD project, I will quantify the variance in model output due to modelling decisions. The project will be based on interviews with scientific and non-scientific modellers. The interviews will be used to compute a first in-depth quantification. I will conduct interviews with several non-scientific modellers from several waterboards in the Netherlands. Based on these interviews, I will compute a second quantification. Then, I will conduct interviews with scientists from all over the world to obtain a broad perspective and potentially discern global differences. Finally, I will execute a sensitivity analysis on the model decisions to determine which model decisions influences the model output the most, and where.