Add or Drop Single Terms to a Constrained Ordination Model
Compute all single terms that can be added to or dropped from a constrained ordination model.
## S3 method for class 'cca'
add1(object, scope, test = c("none", "permutation"),
permutations = how(nperm=199), ...)
## S3 method for class 'cca'
drop1(object, scope, test = c("none", "permutation"),
permutations = how(nperm=199), ...)object |
|
scope |
A formula giving the terms to be considered for adding
or dropping; see |
test |
Should a permutation test be added using |
permutations |
a list of control values for the permutations
as returned by the function |
... |
Other arguments passed to |
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 number of permutations 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 permutations should be used if P values really
are important.
Jari Oksanen
add1, drop1 and
anova.cca for basic methods. You probably need these
functions with step and ordistep. Functions
deviance.cca and extractAIC.cca are used
to produce the other arguments than test results in the
output. Functions cca, rda and
capscale produce result objects for these functions.
data(dune) data(dune.env) ## Automatic model building based on AIC but with permutation tests step(cca(dune ~ 1, dune.env), reformulate(names(dune.env)), test="perm") ## see ?ordistep to do the same, but based on permutation P-values ## Not run: ordistep(cca(dune ~ 1, dune.env), reformulate(names(dune.env))) ## End(Not run) ## Manual model building ## -- define the maximal model for scope mbig <- rda(dune ~ ., dune.env) ## -- define an empty model to start with m0 <- rda(dune ~ 1, dune.env) ## -- manual selection and updating add1(m0, scope=formula(mbig), test="perm") m0 <- update(m0, . ~ . + Management) add1(m0, scope=formula(mbig), test="perm") m0 <- update(m0, . ~ . + Moisture) ## -- included variables still significant? drop1(m0, test="perm") add1(m0, scope=formula(mbig), test="perm")
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