"Points" methods

"Points" methods aim on fast quantification of the contribution of each food component to the diet. The basic trick is to avoid the laborious and time consuming complete sorting, counting and weighting of the stomach (gut) content by estimating the amount of each food component according to a rough volumetric scale.

Points methods were developed by fishery biologists (Swynnerton & Worthington 1940, Hynes 1950) and developed further for use in benthic invertebrate studies by Brun (1972), Fratt & Dearborn (1984) and Dearborn et al. (1986).

Modern points methods combine
- information on stomach fullness of a number of individuals i (i = 1, 2, 3.....n)
with
- data on volumetric contribution of each food component j (j = 1, 2, 3.....m) to diet
to obtain reliable estimates of the significance of each food component in the diet.

  
 
Modern points methods measure

- Stomach fullness of a number of individuals i (i = 1, 2, 3.....n)

- Volumetric contribution to diet of each food component j (j = 1, 2, 3.....m)

to obtain reliable estimates of the significance of each food item in the diet.

The resulting data are

- stomach fullness index SFIi for each individual (0<=SFIi<=16), and

- food component point value CPVi,j (0<=CPVj<=16; CPVi,j = 16ind-1)

  
 
Combination of component point value CPV and stomach fullness index SFI

Reliable estimates of food component significance are obtained by weighted component point values WCPVi,j. These are computed by multiplying each CPVi,j by the corresponding SFIi (Dearborn et al. 1986). As both CPV and SFI range between 0 and 16, WCPV ranges between 0*0 and 16*16:

WCPVi,j = CPVi,j * SFIi   (0 <= WCPVi,j <= 256)

Note that the sum of all WCPVi,j of individual i always amounts to 16 * SFIi.

For standardisation WCPVi,j are expressed as a proportion DCi,j of total points assigned to all food components in all individuals weighted by the number of individuals n:

               n  m
   DCi,j = WCPVi,j * 100 * n /    WCPVi,j   [fraction]
                    i=1 j=1

Note that single DCi,j values may be >100 and have no direct ecological significance! They represent between-individuals variability and are required for statistical comparisons.
Average values DCj and their standard deviation are used to describe the diet composition of the population:

n      
                DCj  = DCi,j /  n   [% volume]
i=1       
 
  see computational basics demonstrated with the above example of 2 individuals and 2 food components.
  see example calculation based on real world data using a standardised spread sheet.
  Download computation spread sheet for points method calculation.
 
Problems and advantages of points methods

Measuring food component volumetric contribution or stomach fullness by the points scale is
- a quite subjective procedure, i.e. a specific personal bias will be introduced
- a low resolution approach, as there are only 7 steps on the scale
Nevertheless, this method allows for much more individuals to be analysed per unit of time. Thus by sheer power of numbers
- random noise can be greatly reduced and
- a more representative picture on the population level may be obtained.