The product of two variables that constitutes a new variable and that
captures, if it proves to be significant, the combined effect of the two original variables.
Interaction terms can be created from any two variables. Most commonly, though, they
are created from interacting either two categorical variables, or a categorical variable
and a numerical variable (see chapter 10 for a discussion of such models).
Base Variable
These are the original ”uninteracted” variables from which the interaction
terms were created.]
Quadratic model
Any model made up of a combination of terms of the following forms:
Constants, Constant ⋅ Variable, Constant ⋅ Variable2, Constant ⋅ Var1 ⋅ Var2.
Term
A term is any object added to other objects in a mathematical expression. For
example, in the function shown below, there are three terms: 3x, 2 and 5xy.
Factor
In a mathematical expression, a factor is one quantity (a variable or constant) that is
multiplied with other quantities to make a term. For example, in the function above, the
factors of the term 5xy are 5, x, and y. The factors of the term 3x are 3 and x. The term ”2”
has only one factor, itself.
Factoring
Mathematical/algebraic process of breaking terms into factored form so that several
terms with similar factors can be grouped together. Often, this reveals hidden details of the
model and can aid interpretation.
Self Interaction
An interaction term created by multiplying or interacting a base variable with
itself.
Joint Interaction
An interaction term created by multiplying or interacting two different base
variables.
