Dummy Environmental variables
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Some aspects of the environment cannot be described using continuous variables. For example land use is better described by categorical variables. Categorical variables need to be coded as dummy variables if they are to be used in either CCA or RDA. Dummy variables are binary; they take the value 1 or 0.
For every categorical variable with K categories, only K-1 dummy variables can be included in the analysis (see Linear combinations of environmental variables).
For example, suppose in a study of a chalk stream fauna at 5 sites we categorise the stream bed as unmodified, recently dredged or dredged more than 10 years ago, then these data can be entered as two variables as follows:
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.