The main features of wave data collected by voluntary observing ships (VOS) are described as well as the various proposals of calibration of the raw observations. Three different sources of weather data are compared in a North Atlantic location: VOS data, hindcast model data and altimeter satellite data. Special attention is paid to the differences between VOS and other datasets, on the values of significant wave heights and a seasonal variability is detected on the calculated biases of the VOS data. Furthermore, a discussion is provided on the influence of bad weather avoidance on the VOS statistics and the possible implication in the determination of wave parameters for design purposes.