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If the interaction happens between a continuous and a discrete variable it is (if I'm not mistaken) relatively straightforward. The mathematical expression is:

$\hat Y=\hatβ_0+\hatβ_1X_1+\hatβ_2X_2+\hatβ_3X_1∗X_2+\epsilon$

So if we take my favorite dataset mtcars{datasets} in R, and we carry out the following regression:

(fit <- lm(mpg ~ wt * am, mtcars))

Call:
lm(formula = mpg ~ wt * am, data = mtcars)

Coefficients:
(Intercept)           wt           am        wt:am  
     31.416       -3.786       14.878       -5.298  

am, which dummy-codes for the type of transmission in the car am Transmission (0 = automatic, 1 = manual) will give us an intercept of 31.416 for manual (0), and 31.416 + 14.878 = 46.294 for automatic (1). The slope for weight is -3.786. And for the interaction, when am is 1 (automatic), the regression expression will have the added term, $-5.298*1*\text {weight}$, which will add to $-3.786*\text {weight}$, resulting in a slope of $-9.084*\text {weight}$. So we are changing the slope with the interaction.

But when it is two continuous variables that are interacting, are we really creating an infinite number of slopes? How do express the output without corny sentences like "the slope we would get with cars that weight $0\,\text{lbs.}$, or $1\,\text{lb.}$? For example, take the explanatory variables wt (weight) and hp (horsepower) and the regressand mpg (miles per gallon):

(fit <- lm(mpg ~ wt * hp, mtcars))

Call:
lm(formula = mpg ~ wt * hp, data = mtcars)

Coefficients:
(Intercept)           wt           hp        wt:hp  
   49.80842     -8.21662     -0.12010      0.02785

How do we read the output? There seems to be one single intercept 49.80842, whereas it would make sense to have two different intercepts to give flexibility to the fit, as in the prior scenario (what am I missing?). We have a slope for wt and a slope for hp (-8.21662 -0.12010 = -8.33672, is that right?). And finally the more intriguing 0.02785. So, yes, are we constrained to expressing this with absurd scenarios, such as if we had cars with $1\text{hp}$ we would have a modified slope for the weight equal to $(-8.21662 + 0.02785)*1*\text{weight}$? Or is there a more sensible way to look at this term?

SOLUTION:

[Quick note, safe to skip: I really appreciate the answers and help provided, and will accept - it is rather difficult with such outstanding Answers, though. So please don't take this edit as anything more than a way of sharing what I've been doing for a little while this morning: basically hacking away at the R coefficients until I got what I wanted because despite the generous help provided I still couldn't "see" how one of the coeff's worked. Also, all this pre-emption will be erased shortly.]

We can "prove" how these coefficients "work" by simply taking the first values of mpg, wt and hp, which happen to be for the glamorous Mazda RX4:

credit here

These are:

          mpg cyl disp  hp drat   wt  qsec vs am gear carb
Mazda RX4  21   6  160 110  3.9 2.62 16.46  0  1    4    4

And simply run predict(fit)[1] Mazda RX4, which returns a $\hat y$ value of $23.09547$. No matter what, I have to rearrange the coefficient to get this number - all possible permutations if necessary! No just kidding. Here it is:

coef(fit)[1] + (coef(fit)[2] * mtcars$wt[1]) + (coef(fit)[3] * mtcars$hp[1]) + (coef(fit)[4] * mtcars$wt[1] * mtcars$hp[1]) $= 23.09547$.

The math expression is:

$\small \hat Y=\hat β_0 (=1^{st}\,\text{coef})\,+\,\hatβ_1 (=2^{nd}\,\text{coef})\,*wt \,+\, \hatβ_2 (=3^{rd}\,\text{coef})\,*hp \,+\, [\hatβ_3(=4^{th}\,\text{coef})\, *wt\,∗\,hp]$

So, as pointed out in the answers, there is only one intercept (the first coefficient), but there are two "private" slopes: one for each explanatory variable, plus one "shared" slope. This shared slope allows obtaining uncountably infinite slopes if we "zip" through $\mathbb{R}$ for all the theoretically possible realizations of one of the variables, and at any point we combine ($+$) the "shared" coefficient times the remaining random variable (e.g. for hp = 100, it would be 0.02785 * 100 * wt) with its "private" slope (-8.21662 * wt). I wonder if I can call it a convolution...

We can also see that this is the right interpretation running:

y <- coef(fit)[1] + (coef(fit)[2] * mtcars$wt[1]) + (coef(fit)[3] * mtcars$hp[1]) + (coef(fit)[4] * mtcars$wt[1] * mtcars$hp[1])
identical(as.numeric(predict(fit)[1]), as.numeric(y)) TRUE

Having rediscovered the wheel we see that the "shared" coefficient is positive (0.02785), leaving one loose end, now, which is the explanation as to why the weight of the vehicle as a predictor for "gas-guzzliness" is buffered for higher horse-powered cars... We can see this effect (thank you @Glen_b for the tip) with the $3\,D$ plot of the predicted values in this regression model, which conforms to the following parabolic hyperboloid:

enter image description here

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  • $\begingroup$ Thanks, could you share the code for the 3D plot? $\endgroup$ – Tim M. Schendzielorz May 20 at 16:04
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There seems to be one single intercept 49.80842, whereas it would make sense to have two different intercepts

No, it usually wouldn't make sense to have two intercepts; that only makes sense when you have a factor with two levels (and even then only if you regard the relationship holding factor levels constant).

The population intercept, strictly speaking, is $E(Y)$ for the population model when all the predictors are 0, and the estimate of it is whatever our fitted value is when all the predictors are zero.

In that sense - whether we have factor variables or numerical variables - there's only one intercept for the whole equation.

Unless, that is, you're considering different parts of the model as separate equations.

Imagine that we had one factor with three levels, and one continuous variable - for now without the interaction:

enter image description here

For the equation as a whole, there's one intercept, but if you think of it as a different relationship within each subgroup (level of the factor), there's three, one for each level of the factor ($a$) -- by considering a specific value of $a$, we get a specific straight line that is shifted by the effect of $a$, giving a different intercept for each group.

But now let's consider the relationship with $a$. Now for each level of $a$, if $x$ had no impact, there'd be a very simple relationship $E(Y|a=j)=\mu_j$. There's one intercept, the baseline mean (or if you conceive it that way, three, one for each subgroup -- where the intercept would be the average value in that subgroup).

enter image description here
(nb It may be hard to see here but the means are not equally spaced; don't be tempted by this plot to think of $y$ as linear in $a$ considered as a numeric variable.)

But now if we consider $x$ does have an impact and look at the relationship at a specific value of $x$ ($x=x_0$), as a function of $a$, $E(Y|a=j)=\mu_j(x_0)$ -- each group has a different mean, but those means are shifted by the effect of $x$ at $x_0$.

enter image description here

So that would be one intercept (the black dot if it's the baseline group) ... at each value of $x$.

For each of infinite number of different values that $x$ might take, there's a new intercept.

So depending on how we look at it, there's one intercept, or three, or an infinite number... but not two.

Now if we introduce an $x:a$ interaction, nothing changes but the slopes! We still can conceive of this as having one intercept, or perhaps three, or perhaps an infinite number.


So how does this all relate to two numeric variables?

Even though we didn't have it in this case, imagine that the levels of $a$ were numeric and that the fitted model was linear in $a$ (perhaps $a$ is discrete, like the number of phones owned collectively by a household). [i.e. we're now doing what I said earlier not to do, taking $a$ to be numeric and (conditionally) linearly related to $y$]

Then we'd still have one intercept in the strict sense, the value taken by the model when $x=a=0$ (even though neither variable is 0 in our sample), or one for each possible value taken by $a$ (in our sample, three different values occurred, but maybe 0, 4, 5 ... are also possible), or one for each value taken by $x$ (an infinity of possible values since $x$ is discrete). It doesn't matter if our model has an interaction, it doesn't change that consideration about how we count intercepts.


So how do we interpret the interaction term when both variables are numeric?

You can consider it as providing for a different slope in the relationship between $y$ and $x$, at each $a$ (three different slopes in all, one for the baseline and two more via interaction), or you can consider it as providing for a different slope between $y$ and (the now-numeric) $a$ at each value of $x$.

Now if we replace this now numeric but discrete $a$ with a continuous variate, you'd have an infinite number of slopes for both one-on-one relationships, one at each value of the third variable.

You effectively say as much in your question of course.

are we constrained to expressing this with absurd scenarios, such as if we had cars with 1hp we would have a modified slope for the weight equal to (−8.21662+0.02785)∗1∗weight? Or is there a more sensible way to look at this term?

Sure there is, consider values more like the mean. So for a typical relationship between mpg and wt, hold horsepower at some value near the mean. To see how much the slope changes, consider two values of horsepower, one below the mean and one above it.

Where the variable-values aren't especially meaningful in themselves (like score on some Likert-scale-based instrument say) you might go up or down by a standard deviation on the third variable, or pick the lower and upper quartile.

Where they are meaningful (like hp) you can pick two more or less typical values (100 and 200 seem like sensible choices for hp for the mtcars data, and if you also want to look at something near the mean, 150 will serve quite well, but you might choose a typical value for a particular kind of car for each choice instead)

So you could draw a fitted mpg-vs-wt line for a 100hp car and a 150hp car and a 200 hp car. You could also draw a mpg-vs-hp line for a car that weighs 2.0 (that's 2.0 thousand-pounds) and 4.0 or (or 2.5 & 3.5 if you want something nearer to quartiles).

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  • $\begingroup$ +1 Beautiful post, as always. I need time to read it a few more times, and if you don't mind it, ask some follow-up questions. Right off the cuff, I got a bit stuck with the sentence and plot: "(nb It may be hard to see here but the means are not equally spaced; don't be tempted by this plot to think of y as linear in a considered as a numeric variable.)". Also, I would really appreciate it if you could specifically check whether my concrete reading of the R output (in the places where I have the font in bold) is correct. $\endgroup$ – Antoni Parellada Dec 8 '15 at 4:07
  • $\begingroup$ @Antoni I'm not very happy with it right now, as I don't think it makes the points as clearly as I'd hoped to. Hopefully I will figure out a way to make it more simple. I also hope to show a picture of what a fitted model with numeric-numeric interaction actually looks like - it's a particular kind of ruled surface (in fact, a doubly-ruled surface). $\endgroup$ – Glen_b Dec 8 '15 at 4:09
  • $\begingroup$ @Antoni Oh, actually, this one from wikipedia would do just fine for the case where the main effects are zero. Note that each of those red lines in the two horizontal-axis-directions are straight, even though the surface is curved (i.e. if you slice parallel to the non-vertical axes, you get a line) $\endgroup$ – Glen_b Dec 8 '15 at 4:11
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Output

Coefficients:
(Intercept)           wt           hp        wt:hp  
   49.80842     -8.21662     -0.12010      0.02785  

How do we read this output? ... We have a slope for wt and a slope for hp (-8.21662 -0.12010 = -8.33672, is that right?).

Nope. Some calculus should confirm that the derivative of mpg with respect to wt, which is the only kind of slope you should be interested in, is

-8.21662 + 0.02785 x hp

That whole expression is the expected increase in mpg from increasing wt by one. Similarly the derivative of mpg with respect to hp is

-8.21662 + 0.02785 x wt

because interactions are symmetric.

So, yes, are we constrained to expressing this with absurd scenarios, such as if we had cars with 1hp we would have a modified slope for the weight equal to (−8.21662+0.02785) * 1 * weight? Or is there a more sensible way to look at this term?

I'm not sure what you mean here. If you're worried about talking about cars with zero hp or wt or whatnot, then it's probably just easier to redefine those variables to have a zero that's more meaningful.

For example, subtract the Mazda RX4 from each row. Now you have a rotary-engined early seventies zero point to compare to. That is, you are no longer considering cars with an hp of one, but rather cars an hp one higher than that of a Mazda RX4.

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  • $\begingroup$ This is so close to what I was looking for in terms of a concrete answer... Thanks. Just bear with me... Can you explain the meaning of the hp value in the output -0.12010 ? Also, -8.21662 + 0.02785 x hp looks like an intercept and a slope, but we already have an intercept (49.80842)... Should there be a parenthesis, like in (-8.21662 + 0.02785) x hp to denote that the overall slope is modified by the "interaction slope" to end up with -8.21662 + 0.02785 = -8.18877 as the final slope? $\endgroup$ – Antoni Parellada Dec 8 '15 at 5:45
  • $\begingroup$ 1. 'hp' just indicates the value of the hp variable for whatever case we're looking at. Consequently, there are as many different wt slopes as there are distinct values of hp. 2. It does look like a slope and intercept doesn't it? That's not accidental; we are using hp to 'predict' what the coefficient is going to be. $\endgroup$ – conjugateprior Dec 8 '15 at 15:10
  • $\begingroup$ 3. We definitely do not want (-8.21662 + 0.02785) x hp. We want (-8.21662 + (0.02785 x hp)). The coefficient on the interaction is telling us how much larger (or smaller) than -8.21662 the slope should be, as a function of hp. Here we get a less negative slope on wt the higher the hp. It may be helpful to write out the linear predictor equation and differentiate to confirm. $\endgroup$ – conjugateprior Dec 8 '15 at 15:12
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The easiest way is to have a look at different quantiles of one of your variables of interest, depending on your research question. Assuming that you want to know the effect of one additional unit of horse power on the miles per gallon a car uses, you look at the distribution of the weight and use e.g. the percentiles 1, 10, 25, 50, 75, 90 and 99. For each of these percentiles you compute the effect of hp on mpg.

Concerning interaction terms and parent variables (variables which are interacted) Chipman put forward the Heredity principle (http://arxiv.org/abs/bayes-an/9510001). Basically he differentiates between strong and weak heredity, depending on how much parent variables are included in the regression. From a statistical point of view it would also be correct to just add the interaction term, without the variables themselves. But this makes the results hard (/impossible) to interpret.

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