U' = n^n^-U 



After calculation of U, the calculation of U' is made. The larger 

 of two values is used as the test statistic. It is compared to a table 

 of U values, and if the calculated value exceeds the tabled value at 

 the selected level of probability, this is taken as evidence of a 

 significant difference between the two groups. 



5. Spearman Rank Correlation . 



This is a nonparametric test which measures the association or 

 correlation between two variables. It is the nonparametric equivalent 

 of the parametric correlation coefficient and is used if the variables 

 under consideration are not normally distributed. 



The test was used because of the assumption that the abundances of 

 clams and the sediment parameters were normally distributed. 



The formula for this test is: 



' 6J:dj^2 



n3-N 



where : 



d.^ = the squared difference in two ranks 



N = total number of paired observations 



The Spearman rank coefficient, r , calculated as above, is then 

 compared to a tabled value. If the calculated r exceeds the tabled 



r value at the appropriate level of probability it is taken as evidence 

 of a significant correlation. 



6. Linear Regression . 



This is a parametric analysis which measures how much increase or 

 decrease in one factor may be expected from a unit increase in the other. 

 The results of a regression analysis are given in the form of an 

 equation which relates one variable to the other. The general form of the 

 regression equation is: 



Y = a + b X 



yx 



where : 



64 



