1923 1 Esterly: Marine Copepoda at La Jolla 423 



average number of animals per haul for two years as it is given in 

 the same column is determined. This process gives a table consisting 

 of six columns and twenty-four lines. (2) The horizontal sum of each 

 line (step 1) is divided by 6, which gives the coefficients. (3) Each 

 of the original entries (table 1) is multiplied by the coefficient for the 

 corresponding month. These products are the corrected values. This 

 method constitutes a rough check against the first one. 



TABLE 2 



Corrected Data for Acartia tonsa, Comparing the Results Obtained by the 



Methods of "Means of Ratios" and "Ratios of Means." 



There is also a comparison of the first with the second year. 



Number of animals per haul Mean 



S a.m. Noon 4 p.m. S p.m. Midnight 4 a.m. abund. 



Means of ratios : 



First year 81 25 97 184 478 377 207 



Second year 203 41 165 479 684 651 370 



Ratio of means: 



First year 49 IS 48 118 147 146 88 



Second year 68 19 38 124 133 152 89 



Two-year average: 



Means of ratios 142 33 131 331 581 514 



Ratio of means 59 19 43 121 140 149 



The mean annual abundance and the mean monthly abundance 

 of the copepods may both be obtained from the values as corrected by 

 either of the methods. The vertical sum of each of the six columns 

 of corrected values is divided by 24, and the sum of the quotients is 

 then divided by 6 to give the mean annual abundance. The mean 

 monthly abundance is obtained by dividing the mean annual abundance 

 by the coefficient for the corresponding month as derived by step 2 in 

 either method. 



The results obtained by using the two methods check fairly well, as 

 differences are usually in the same direction. In table 2 there appear 

 the final results for Acartia tonsa as obtained by each of the methods, 

 but for the other forms only the averages for the two methods have 

 been set down. 



The probabilities in table 6 were determined from the corrected 

 results by a method worked out by McEwen. The method has not 

 been published, but it has been briefly described (McEwen, 1920, pp. 

 263, 273). The figures giving the probability indicate the number 

 of times a difference would occur if the results were due to chance. 

 When the values are smaller there is, however, less likelihood that 



