# An Illustrated Introduction to Optimal Treatment Rules

This post walks through the basic statistical intuition for Optimal Treatment Rules (OTRs) for applied scientists. Each concept is accompanied by a small visual to aid in comprehension.

Optimal treatment rules (OTRs) is a fast-growing topic in the medical research community. A treatment rule is a decision for treatment based upon a patient’s characteristics. The intuition behind this is that not all patients will respond to a treatment in the same way. We can exploit these heterogeneous effects and develop personalized rules which provide benefit to the most individuals.

Developing and optimizing treatment rules is rooted in principles of causal inference, or using data to inform us about what would have happened in a hypothetical world in which different interventions had occurred.

Although this post is introductory, it assumes basic knowledge in causal inference, such as counterfactual outcomes, assumptions for causal identification, Average Treatment Effect, and G-computation. If you’re rusty on these concepts, I recommend reading the freely available introductory chapters of Miguel Hernan and James Robins’ What If book.

# 🗺️ The Big Picture of OTRs

Let’s first think through the concepts of OTRs using mathematical notation.

1. We will start with a matrix of observed data $$O$$ which includes our outcome $$Y$$, the exposure (i.e. treatment, medicine, etc.) we want to study $$A$$, and other covariates $$W$$. We can denote these random variables as $$O = (W, A, Y)$$.

1. Now consider that we’ve created some function, $$d$$, which takes baseline confounders $$W$$ and outputs a treatment assignment $$A$$. We can write this mapping function, or treatment rule, as:

$d: W \rightarrow A$

1. We can then think about the counterfactual outcome1 for each observation under the treatment rule $$d$$. Let’s denote this vector of counterfactual outcomes as $$Y(d)$$.

1. The best treatment rule will maximize the expected counterfactual outcome, or $$E[Y(d)]$$, across the entire population. We can write that using $$\mathop{\mathrm{arg\,max}}$$, which means we want to know which argument will return the highest value of a function. In this case, we want to know what treatment rule $$d$$ returns the highest expected value of the counterfactual outcome, $$E[Y(d)]$$.

$\mathop{\mathrm{arg\,max}}_d E[Y(d)]$

1. We’ll call whatever function $$d$$, or $$d(W)$$, that maximizes this expected counterfactual outcome for the population $$d^*(W)$$. This $$d^*(W)$$ is our Optimal Treatment Rule.

# 📈 Estimating the OTR

There are many ways to estimate $$d^*(W)$$. One of the most common ways begins by estimating the Conditional Average Treatment Effect (CATE).

You have probably heard of the Average Treatment Effect (ATE), which is the mean difference in outcomes in a world in which every unit receives the exposure compared to a world in which no unit receives the exposure. In potential outcomes notation, $$ATE = E[Y^1-Y^0]$$. The CATE is the same formula and description, but within covariate strata $$W$$.

$CATE = E[Y^1-Y^0|W]$

Under standard causal assumptions2, the CATE for a binary exposure is identifiable under the following formula:

$\mathrm{CATE}(W) = E[Y|A=1, W] - E[Y|A=0, W]$

We could estimate the CATE using G-computation. If you’d like a review on G-computation, check out this visual guide.

1. Fit a regression for $$E[Y|A,W]$$.

1. Obtain predicted estimates for $$Y$$ on datasets where all observations are changed to have $$A=1$$ and $$A=0$$ using the model fit from Step 1.

$\hat{E}[Y|A=1, W]$

$\hat{E}[Y|A=0, W]$

1. Compute the difference in the predicted $$Y$$’s for the two datasets from Step 2.

$\widehat{CATE}(W) = \hat{E}[Y|A=1, W] - \hat{E}[Y|A=0, W]$

Then, we could say our OTR is to give treatment if the value of $$CATE(W)$$ for that person is positive, indicating a positive effect of treatment on the outcome $$Y$$. Likewise, if the value is negative or 0, indicating a negative or neutral effect on the outcome $$Y$$, that unit would not receive treatment under the OTR.

$\mathrm{OTR}(W) = \mathbb{1}{ \{\mathrm{CATE}(W) > 0} \}$

# 🖥️ R simulation

Let’s take a look an R simulation for the simple estimation of $$d^*(W)$$ we just described. We can first simulate data of n = 500 rows, where we have only one confounder W, a binary treatment A which depends on W, and an outcome Y which is continuous and depends on W and A.

n <- 500
W <- runif(n, 1, 99)
A <- rbinom(n, 1, prob = abs(W/100))
Y <- rnorm(n, 10) + rnorm(n, 2*A) + rnorm(n, 50*W) - rnorm(n, .1*A*W)
df <- data.frame(W, A, Y)

We’ll run a regression for a saturated linear regression model of $$E[Y|A,W]$$, then obtain predictions on datasets where A is changed to 1 and 0 for all rows. We can then compute the CATE as the difference between these predictions.

fit <- glm(Y~A*W)
E_Y1 <- predict(fit, newdata = data.frame(A = 1, W))
E_Y0 <- predict(fit, newdata = data.frame(A = 0, W))
CATE <- E_Y1 - E_Y0

Finally, our optimal treatment rule will be to treat any unit with CATE > 1. We can visually see there is benefit for about 1/4 of units in our simulated population.

library(tidyverse)
data.frame(CATE) |>
mutate(d_star = ifelse(CATE > 0, "Treat", "Do not treat")) |>
ggplot(aes(CATE, fill=d_star)) +
geom_histogram(binwidth=.2) +
theme_bw() +
scale_fill_manual(values = c("#f2696f","#4984b0")) +
labs(x="CATE(W)", y = "Count", fill = "Treatment Rule", title="Distribution of CATE(W)")

# Improving estimation of $$d^*(W)$$

There are much more advanced ways to estimate $$CATE(W)$$ using data-adaptive machine learning techniques and improved robustness properties. Of particular interest is creating doubly-robust estimators which do not rely on getting both the estimation of $$d(W)$$ and $$E[Y|W,A]$$ correct for the final $$d^*(W)$$ to be correct.

We could also estimate $$d^*(W)$$ directly instead of first estimating $$CATE(W)$$. We can extend either of these ideas to longitudinal settings, studies with clustering, etc. I’ve listed some of the resources I am reading to learn about OTRs below. As always I welcome feedback and/or suggestions of additional resources I can include.

These concepts are introductory, so any paper on “optimal treatment rules”, “individualized treatment rules”, or “heterogeneous treatment effects” should review the ideas discussed here in their introductions.

• For example, this recent paper by Edward Kennedy discusses one way to evaluate the CATE using doubly robust methods, and gives several other foundational papers in the introduction.

• This R blog post about heterogeneous treatment effects also may be useful for thinking through these issues with real data.

I’ll continue to add resources to this list as I discover them. Please reach out if you have recommendations of papers or tutorials (yours or others!) to add to this list.

## Acknowledgments

Thanks to my colleague Iván Díaz for explaining OTRs to me and reviewing this post.

1. Recall that a counterfactual describes a hypothetical world where a unit received a certain intervention or treatment, which might be different from the treatment they actually received↩︎

2. This post is focused on estimation and therefore does not detail the requirements for causal identification, but here I refer to the assumptions of consistency, exchangeability, and positivity.↩︎

##### Katherine Hoffman
###### Research Biostatistician

I am passionate about meaningful, reproducible medical research.