SEA-FISHERIES LABORATORY. 



169 



results of the hauls, we find that three of the series are within 

 the range and three are outside it, and two of the latter (3,880 

 and 10,020) are very considerably beyond the limits of the 

 probable error of the experiment. 



The Diatoms for the other hauls give much the same result 

 when treated in the same manner — that is, roughly 50 per 

 cent., or rather more, of the observed variation in the catches 

 is not covered by the calculated range of error of the experiment. 



In the following tables the three principal groups of the 

 plankton, Diatoms, Dinoflagellates and Copepoda,and also three 

 prominent organisms — one from each of these groups — are 

 shown for all seven series of hauls treated as in the case of 

 the Diatoms of April 3rd discussed above, and giving in each 

 case the figures necessary to make a comparison between the 

 range of variation in the catches and the calculated range of 

 error : — 



than the mean. Then the first process is to find the " standard deviation. 

 Take, for example, the six successive hauls on April 3rd : — 



j 



No. of organisms. ' Frequency. 



Deviation from 

 Mean. 



A2 



3.880 

 6,670 

 8,770 

 9,220 

 9,770 

 10,020 





-4,175 

 -1,385 

 + 715 

 + 1,165 

 + 1,715 

 + 1,965 



17,430,580 

 1,918,225 

 511,225 

 1,357,224 

 2,941,224 

 3,861,225 



48,330 

 Mean = 8,055 



6 







28,019,703 



Find 



V 



S Aa 

 ~6~ 



2,161 = standard deviation. 



Next find 0-6745 X standard deviation = probable error = 1,458. 

 The range of error = mean + probable error = 8,055 + 1,458 

 = 6,600 to 9,500 (approximately). 



The convention is to regard this as the catch, that is the variations 

 between 6,600 and 9,500 are not of any significance — any number 

 between these limits is equally permissible, that amount of variation being 

 possibly due to the unavoidable errors of the experiment. 

 L 



