"Points" methods - Food Item Contribution		  
 
Measurement principles

Volumetric contribution of one food item is judged according to a discrete 7-step scale ranging from "nil" to "all". This scale translates into relative values between 0% and 100% represented by corresponding "points".

Qualitative Nil Traces Little Some Half 3/4 All
% 0 6.3 12.5 25 50 75 100
Points 0 1 2 4 8 12 16
 
 
Volumetric contribution of each food component to diet

The stomach of a specimen i contains a certain volume of food.
This volume of food is composed of a number of food components j (j = 1, 2, 3.....m).
Each food component j receives a component point value CPVi,j on the points scale from 0-16 according to its contribution to total stomach content.
The sum of all component point values CPVi,j of individual i must equal 16 (= 100% = All)

CPVi,j = 16 points for 1 individual
 
 
Example

Imagine a carnivorous polychaete which feeds on two prey species, an amphipod and an isopod. In the gut of three individuals examined, we found the following number of food items:

  Food component 1
Amphipod		 Food component 2
Isopod		 Total number of items 
Individual 1 21 none 21
Individual 2 5 5 10
Individual 3 none 3 3

Assuming that all amphipods and isopods are of about the same volume and body mass, these data trranslate into the following CPVi,j table:

  Food component 1 Food component 2 Sum
Individual 1 16 0 16
Individual 2 8 8 16
Individual 3 0 16 16
Average 8 8 -

  

  
 
This table indicatess that using component point values as stand-alone measures of diet composition may lead to serious bias.

The average CPV values indicate that amphipods and isopods are of equal significance because average CPV of both components is 8.
In total, however, we counted 26 ampipods and 8 isopods (see first table) thus it seems obvious that food component 1 is much more important than food component 2.