On the study of the wave induced variable of a ship, the full scale measurement have been developed a many useful and remarkable success. Because of the measuring period is short in such measurement, the wave condition encountered by the ship may deviate from that to be encountered in her life time. So, it is feared that the estimated extreme value of the response from the result of the measurement will include error which is not to be neglected. Authors research the observed wave conditions of 62 voluntary merchant ships in North Pacific Ocean for three years (1976-78). It is clarified by our research that in the case of such short term operation the difference of wave condition between ships according to variability of service condition is remarkable in comparison with the difference between random samples from the same population. Regarding each wave condition observed by a voluntary ship as that observed by a ship on which the response is measured, the reliability of the extreme value over her life time estimated from result of the measurement are studied. The estimated values scatter over wide range, and moreover, the reliability of the value falls off as the length of the ship becomes longer. So, it is concluded that to estimate the correct extreme value of the wave induced variable from the measurement for such years may be difficult especially in the case of a large ship. Authors analyse the effect of wave condition on the extreme, and suggest a new method to estimate the extreme from the result of the measurement with higher reliability.