# Interaction term in multivariate polynomial regression

I'm looking for answer for the question about multivariate polynomial regression. I can't find a clear explanation of when an interaction term is necessary.

Some sources say that the estimated model of a complete second degree polynomial regression model in two variables $x_{1}$, $x_{2}$ may be expressed as

$$\hat{y} = b_{0} + b_{1}x_{1} + b_{2}x_{2} + b_{3}x_{1}^{2} + b_{4}x_{2}^{2} + + b_{5}x_{1}x_{2}$$

and others don't consider interaction term $x_{1}x_{2}$...

And how does it look when I have for example $d=4$ and $p=3$, a third degree polynomial of four independent variables?

• What are d & p here? – gung - Reinstate Monica May 8 '16 at 15:52
• Sorry! p is a degree of polynomial and d is dimension of a space:) – madzioch May 8 '16 at 15:55
• Please, edit your question in order to correct your typos and include that information about p and d. – Marcelo Ventura May 9 '16 at 17:13
• Sorry for the late response, but... There is free online software for fitting multivariate polynomial regressions that uses various tests to select which terms contribute significantly to the model. You can test individual terms using t-statistics, or a variety of global statistics (MSE, adjusted R^2, F, AIC, BIC). It also facilitates cross-validation, which strongly helps avoid overfitting. The software is at www.TaylorFit-RSA.com. – Dave Vaccari Sep 18 '18 at 16:16

Following the guidelines of Blackwell ("all models are wrong, but some are useful "), you have no obligation of including an interaction term, since:

1. you may not afford including an interaction term due to a small number of observations in your dataset;
2. you may have too much noise in your phenomenon so that including interaction terms can be futile, since you seldom will get them statistically significant; and
3. higher degree interaction terms (like $x_1^3 x_5^2 x_9^5$, for instance) will very, very, very rarely have any meaningful interpretation in terms of your real phenomenon (and it only gets worse if your $y$ is multivariate).

OK, in strict mathematical speak, you can (perhaps ought to) include them, but considering strictly modeling issues, most of time it will not make sense for your first approach to a phenomenon, albeit it can and will make more sense in the course of a series of improvements of an initial model, like, for instance, in the regression of the concentration of one product in function of the concentration of several reagents and catalysts in a very complex chemical reaction.

But please notice that all of it will make sense only if your experiments are extremely well controlled and if you have a horribly awful lot of observations, both conditions more compatible with a rather long series of experiments in a well consolidated research, not in a very first attempt to get to understand, say, a social phenomenon.

And, last but not least, as asked, $$y = \sum_{i_1=0}^{3} \sum_{i_2=0}^{3-i_1} \sum_{i_3=0}^{3-i_1-i_2} \sum_{i_4=0}^{3-i_1-i_2-i_3} \beta_{i_1i_2i_3i_4} x_1^{i_1}x_2^{i_2}x_3^{i_3}x_4^{i_4} +\varepsilon,$$ which, according to my calculations, will be a hell of a long expression. =)

Something like $$y = \beta_{0000} + \beta_{1000} x_1+ \beta_{0100} x_2+ \beta_{0010} x_3+ \beta_{0001} x_4+ \beta_{2000} x_1^2+ \beta_{0200} x_2^2+ \beta_{0020} x_3^2+ \beta_{0002} x_4^2+ \beta_{1100} x_1x_2+ \beta_{1010} x_1x_3+ \beta_{1001} x_1x_4+ \beta_{0110} x_2x_3+ \beta_{0101} x_2x_4+ \beta_{0011} x_3x_4+ \beta_{3000} x_1^3+ \beta_{0300} x_2^3+ \beta_{0030} x_3^3+ \beta_{0003} x_4^3+ \beta_{1110} x_1x_2x_3+ \beta_{1101} x_1x_2x_4+ \beta_{0111} x_2x_3x_4+ \ldots+ \varepsilon.$$

Edit: I took the time to write a lazy script in R to generate an expression in LaTeX with all the terms of the sum above, which output this: $$y = \beta_{0000} + \beta_{0001} x_4 + \beta_{0002} x_4^2 + \beta_{0003} x_4^3 + \beta_{0010} x_3 + \beta_{0011} x_3 x_4 + \beta_{0012} x_3 x_4^2 + \beta_{0020} x_3^2 + \beta_{0021} x_3^2 x_4 + \beta_{0030} x_3^3 + \beta_{0100} x_2 + \beta_{0101} x_2 x_4 + \beta_{0102} x_2 x_4^2 + \beta_{0110} x_2 x_3 + \beta_{0111} x_2 x_3 x_4 + \beta_{0120} x_2 x_3^2 + \beta_{0200} x_2^2 + \beta_{0201} x_2^2 x_4 + \beta_{0210} x_2^2 x_3 + \beta_{0300} x_2^3 + \beta_{1000} x_1 + \beta_{1001} x_1 x_4 + \beta_{1002} x_1 x_4^2 + \beta_{1010} x_1 x_3 + \beta_{1011} x_1 x_3 x_4 + \beta_{1020} x_1 x_3^2 + \beta_{1100} x_1 x_2 + \beta_{1101} x_1 x_2 x_4 + \beta_{1110} x_1 x_2 x_3 + \beta_{1200} x_1 x_2^2 + \beta_{2000} x_1^2 + \beta_{2001} x_1^2 x_4 + \beta_{2010} x_1^2 x_3 + \beta_{2100} x_1^2 x_2 + \beta_{3000} x_1^3 + \varepsilon$$

The script itself is

d <- 3
formula <- "$$y = " %% <- paste0 for (i1 in 0:d) { for (i2 in 0:(d-i1)) { for (i3 in 0:(d-i1-i2)) { for (i4 in 0:(d-i1-i2-i3)) { formula <- formula %% "\\beta_{" %% i1 %% i2 %% i3 %% i4 %% "}" %% (if (i1>0) (" x_1" %% if (i1>1) ("^" %% i1))) %% (if (i2>0) (" x_2" %% if (i2>1) ("^" %% i2))) %% (if (i3>0) (" x_3" %% if (i3>1) ("^" %% i3))) %% (if (i4>0) (" x_4" %% if (i4>1) ("^" %% i4))) %% " + " } } } } formula <- formula %% "\\varepsilon$$"
cat(formula)


At last, notice that it generated no fourth degree terms, like $\beta_{1111} x_1 x_2 x_3 x_4$, since you required a third degree polynomial ($d=3$) with four variables ($p=4$).

• Thank you Marcelo Ventura! Unfortunately I can't voting :( I would like to ask if you would be so kind as to wrote formula for d-degree polynomial with p variables? What do You think about this kind calculation during local polynomial regression? Is it really necessary? I'm not sure... – madzioch Jun 6 '16 at 17:26

One could use the model with or without interaction terms, depending on whether one expected them to be useful. In practice, I would expect any description of a polynomial regression model to be clear about whether or not interaction terms were included. If no mention is made of them, I would assume not.

• Thanks! But... what do You think about adding interaction term during local polynomial regression? Is it necessary, or not? – madzioch May 8 '16 at 21:32
• I think it depends on the problem. Interaction terms are useful in some cases and not useful in others. (If you felt my answer was satisfactory, remember to accept it by clicking on the check mark under the voting arrows.) – Kodiologist May 9 '16 at 0:09

One consideration in such problems is the interpretability of effects. The interpretation of higher order terms as well as their interactions is somewhat involved, but important to understand. These are discussed in other CV posts, so do check them.

The consensus is that, with higher order terms as well as their interactions, you may fit interactions as needed provided that the order does not exceed the order of the main effects. This preserves the interpretation of the effect of product terms as a difference in differences. So in your example, it makes sense to include the $x_1 x_2$ product term since you have their respective main effects $x_1$ and $x_2$. Furthermore, you could also include $x_1 x_2^2$, $x_1^2 x_2$, and/or $x_1^2 x_2^2$ since they are all nested within the lower level terms. One would not, however, adjust for (say) $x_1^3 x_2$ without first adjusting for the $x_1^3$ main effect.

The number of possible product terms in a model controlling for polynomial orders quickly becomes untenable. A "semi-saturated" model for the outcome is quickly approached with relatively few effects. For instance, if $d$ is the polynomial order of $x_1$ and $q$ is the polynomial order of $x_2$ then there are $d + q$ main effects and $d \times q$ possible product terms. Fitting semi-saturated models has a purpose in some estimation problems (like robust variance estimation), but you quickly approach an issue of overfitting in most settings. Using an information criterion like the BIC can help to select a model with a lower number of overall effects, but high predictive accuracy and external validity.