Table 4 gives the data needed for typical significant heights. In 

 an actual observation the number obtained is the average height of 

 all waves in excess of a certain height, and the table then gives the 

 true significant height, the true average height of the total sample, 

 and the percent of waves omitted. 



In each entry, the average height of all waves is less than the average 

 height of all waves in excess of some fixed height. Thus the tendency 

 to ignore the low waves can make a visual observation quite unreliable 

 unless corrections for the omitted waves are made theoretically. 

 Note also that as the percent of waves omitted becomes smaller the 

 difference between the average of the truncated distribution and the 

 average of the full distribution becomes less and less and less. 



9. An Example 



The data obtained by the USCGC UNIMAK from 1800 to 1900Z on 

 February 14, 1953 give an example of the procedures which can be 

 employed in the use of the theory of a truncated distribution. 



The original raw data were first of all averaged to determine the 

 average height of the reported waves. Then the value of vE was 

 computed. From this and table 1 the theoretical distribution can be 

 compared with the observed values. The result is given in table 5. 



Table 5 

 Data obtained by USCGC UNIMAK 141800Z to 141900Zof February 1953 

 Average Height 15.5 feet (uncorrected) 

 Significant Height 24.5 feet (uncorrected) 



Error 



-5 



+4 

 + 1 

 

 -5 

 +1 

 +2 

 +5 

 + 1 

 -4 



24 



