Arguments
object A constrained ordination object from cca, rda or capscale.
scope A formula giving the terms to be considered for adding or dropping; see add1
for details.
test Should a permutation test be added using anova.cca.
permutations a list of control values for the permutations as returned by the function how, or
the number of permutations required, or a permutation matrix where each row
gives the permuted indices.
... Other arguments passed to add1.default, drop1.default, and anova.cca.
Details
With argument test = "none" the functions will only call add1.default or drop1.default. With
argument test = "permutation" the functions will add test results from anova.cca. Function
drop1.cca will call anova.cca with argument by = "margin". Function add1.cca will implement
a test for single term additions that is not directly available in anova.cca.
Functions are used implicitly in step, ordiR2step and ordistep. The deviance.cca and deviance.rda
used in step have no firm basis, and setting argument test = "permutation" may help in getting
useful insight into validity of model building. Function ordistep calls alternately drop1.cca
and add1.cca with argument test = "permutation" and selects variables by their permutation
P-values. Meticulous use of add1.cca and drop1.cca will allow more judicious model building.
The default perm.max is set to a low value, because permutation tests can take a long time. It should
be sufficient to give a impression on the significances of the terms, but higher values of perm.max
should be used if P values really are important.
Value
Returns a similar object as add1 and drop1.