How do you make a contrast matrix in R?
Construct a User-Specified Contrast Matrix
- Description. Construct a user-specified contrast matrix.
- Usage. make.contrasts(contr, how.many = ncol(contr))
- Arguments. contr.
- Details. This function converts human-readable contrasts into the form that R requires for computation.
- Value. make.
- Author(s) Gregory R.
- See Also.
- Examples.
What is contrast matrix in R?
These functions are used for creating contrast matrices for use in fitting analysis of variance and regression models. The columns of the resulting matrices contain contrasts which can be used for coding a factor with n levels. The returned value contains the computed contrasts.
What is a design matrix in R?
matrix creates a design matrix from the description given in terms(object) , using the data in data which must supply variables with the same names as would be created by a call to model. If data is a data frame, there may be other columns and the order of columns is not important.
What does contrasts mean in R?
Contrasts can be used to make specific comparisons of treatments within a linear model. One common use is when a factorial design is used, but control or check treatments are used in addition to the factorial design. They also allow for an F-test for multi-line contrasts, for example when testing within groups.
What package is contrasts in R?
The packages lsmeans and multcomp allow for unlimited tests of single-degree contrasts, with a p-value correction for multiple tests. They also allow for an F-test for multi-line contrasts, for example when testing within groups.
How are contrasts used in a design matrix?
If there are any factors in terms in the model, there is an attribute “contrasts”, a named list with an entry for each factor. This specifies the contrasts that would be used in terms in which the factor is coded by contrasts (in some terms dummy coding may be used), either as a character vector naming a function or as a numeric matrix.
How to create a contrast matrix in limma?
This example from limma is a bit confusing, because what they do is: they first create a contrast matrix, then they build a design matrix, and then they build another contrast matrix that they actually use.
When to use the contrasts function in rdocumentation?
If contrasts are not set for a factor the default functions from options (“contrasts”) are used. A logical vector x is converted into a two-level factor with levels c (FALSE, TRUE) (regardless of which levels occur in the variable). The argument contrasts is ignored if x has a matrix contrasts attribute set.
What’s the difference between a model and a matrix?
model.matrix creates a design (or model) matrix, e.g., by expanding factors to a set of dummy variables (depending on the contrasts) and expanding interactions similarly.