The uncertainties in the virtual design stages of the product development chain are considered. The current computer capacity enables the processing of complex models to handle these steps. However, in the present work it is shown that the prediction capacity is not always favoured by an increased model complexity. In fact, it is observed that the prediction capacity deteriorates when the complexity exceeds a certain level. An example is shown and its implication for fatigue design are discussed. Key words: fatigue, model complexity, prediction uncertainty