Canonical Correspondence Analysis
34
Site
unmodified
recently
dredged
1
0
0
2
1
0
3
1
0
4
0
1
5
0
1
Site 1 was dredged more than 10 years ago, Sites 2 & 3 are unmodified and sites 4 &
5 have been recently dredged. Note that we do not need to explicitly include the
category  dredged more than 10 years ago . Indeed, you must not include it if you
are to avoid the environmental variables becoming linear combinations of each other.
See also:
Selecting Environmental variables
Linear combinations of environmental variables
Transforming environmental variables
Circular environmental variables
7.5
Transforming environmental variables
Palmer (1993)
 recommends logarithmic transformations of soil nutrient
concentrations when undertaking vegetational analysis "because a 1 unit difference in
nutrient concentration is probably much more important at low concentrations than it
is at high concentrations".
Linear transformations of variables will have no effect on the results of a CCA.
Therefore, changing the units of a variable (e.g. centimeters to millimeters) or the
base point (e.g. elevation above a fixed point to elevation above sea level) will have
no effect.
See also
Data transformations
Selecting Environmental variables
Linear combinations of environmental variables
Dummy Environmental variables
7.6
Multicollinearity
If the environmental variables are highly correlated then results obtained by any
multiple regression method can be highly unreliable. ECOM tests for appreciable
collinearity between the environmental variables and will give a warning message if
appreciable levels of collinearity are present.
Copyright 2004  PISCES Conservation Ltd





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