96
Community Analysis Package
18
Association analysis
The association between species (rows) in the working data is shown by selecting
Association Chi squared from the drop down menus. For each species pair the
contingency table of presence/absence is used to calculate a Chi squared value.
For measures of similarity between samples based on species presence absence
the observations can be summarised in a simple frequency table:
Sam
Species Present
Sample 2
Species Present
a
Species  Absent
c
where the number of species present in both samples is a, the number of species
present in sample 1 but missing from sample 2 is b, the number of species
missing in sample 1 but present in sample 2 is c and the number of species
missing from both samples is d. The total number of species, N, is therefore
a+b+c+d.
The Chi squared value for each contingency table is calculated in the normal
fashion using the Yate s correction for small numbers of observations.
The Chi squared values are presented in a grid displayed by clicking on the Chi 
squared tab with the following display conventions:
Chi squared values, which indicate a significant degree of positive or negative
association between the species at the 5% level, are shown in bold. Positive
associations are displayed in blue and negative associations as negative
numbers in red. If either of the two species is present in every sample the
contingency table is meaningless and a value of 99 is displayed.
Please note that association analysis will give misleading results if the data
comprises many zeros caused by low sampling effort rather than true species
presence/absence.
Copyright 2004, PISCES CONSERVATION LTD





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