This paper presents the results of statistical analyses on the difference between measured and calculated fatigue rates for an ocean-going container vessel. Data from a hull monitoring system installed on-board this ship provides time-series of observed fatigue rates in actual operation, sea- and weather conditions. In addition, a hydrodynamic model has been used to calculate fatigue rates for selected locations in the mid-ship section due to vertical bending moment only, corresponding to experienced weather conditions over the same time period. The measured fatigue rates are then compared to the calculated fatigue rates and statistical regression models are established to explain the differences as a function of selected explanatory variables. This can then be used to correct for biases in the numerical models. Overall, the results indicate that the models perform reasonably well and are able to describe much of the variation in the difference. However, the models are found to not generalize very well and it may be challenging to find models for a whole fleet of ships.