# Double Machine Learning

Double machine learning, also called debiased or orthogonalized machine learning, enables estimating causal effects when the dimensionality of the covariates is too high for linear regression or the model does not assume a parametric form. In other words, when the relathionship between the treatment or covariates and outcome is nonlinear and we do not know the functional form.

Note

Chernozhukov, Victor, Denis Chetverikov, Mert Demirer, Esther Duflo, Christian Hansen, Whitney Newey, and James Robins. "Double/debiased machine learning for treatment and structural parameters." (2018): C1-C68.

## Step 1: Initialize a Model

The DoubleMachineLearning constructor takes at least three arguments, an array of covariates, a treatment vector, and an outcome vector.

Tip

You can also specify the following options: whether the treatment vector is categorical ie not continuous and containing more than two classes, whether to use L2 regularization, the activation function, the validation metric to use when searching for the best number of neurons, the minimum and maximum number of neurons to consider, the number of folds to use for cross validation, the number of iterations to perform cross validation, and the number of neurons to use in the ELM used to learn the function from number of neurons to validation loss. These arguments are specified with the following keyword arguments: t_cat, regularized, activation, validation_metric, min_neurons, max_neurons, folds, iterations, and approximator_neurons.

# Create some data with a binary treatment
X, T, Y =  rand(100, 5), [rand()<0.4 for i in 1:100], rand(100)

# We could also use DataFrames
# using DataFrames
# X = DataFrame(x1=rand(100), x2=rand(100), x3=rand(100), x4=rand(100), x5=rand(100))
# T, Y = DataFrame(t=[rand()<0.4 for i in 1:100]), DataFrame(y=rand(100))

dml = DoubleMachineLearning(X, T, Y)

## Step 2: Estimate the Causal Effect

To estimate the causal effect, we call estimatecausaleffect! on the model above.

# we could also estimate the ATT by passing quantity_of_interest="ATT"
estimate_causal_effect!(dml)

# Get a Summary

We can get a summary that includes a p-value and standard error estimated via asymptotic randomization inference by passing our model to the summarize method.

Calling the summarize method returns a dictionary with the estimator's task (regression or classification), the quantity of interest being estimated (ATE), whether the model uses an L2 penalty (always true for DML), the activation function used in the model's outcome predictors, whether the data is temporal (always false for DML), the validation metric used for cross validation to find the best number of neurons, the number of neurons used in the ELMs used by the estimator, the number of neurons used in the ELM used to learn a mapping from number of neurons to validation loss during cross validation, the causal effect, standard error, and p-value.

# Can also use the British spelling
# summarise(dml)

summarize(dml)

## Step 4: Validate the Model

We can validate the model by examining the plausibility that the main assumptions of causal inference, counterfactual consistency, exchangeability, and positivity, hold. It should be noted that consistency and exchangeability are not directly testable, so instead, these tests do not provide definitive evidence of a violation of these assumptions. To probe the counterfactual consistency assumption, we assume there were multiple levels of treatments and find them by binning the dependent vairable for treated observations using Jenks breaks. The optimal number of breaks between 2 and num_treatments is found using the elbow method. Using these hypothesized treatment assignemnts, this method compares the MSE of linear regressions using the observed and hypothesized treatments. If the counterfactual consistency assumption holds then the difference between the MSE with hypothesized treatments and the observed treatments should be positive because the hypothesized treatments should not provide useful information. If it is negative, that indicates there was more useful information provided by the hypothesized treatments than the observed treatments or that there is an unobserved confounder. Next, this methods tests the model's sensitivity to a violation of the exchangeability assumption by calculating the E-value, which is the minimum strength of association, on the risk ratio scale, that an unobserved confounder would need to have with the treatment and outcome variable to fully explain away the estimated effect. Thus, higher E-values imply the model is more robust to a violation of the exchangeability assumption. Finally, this method tests the positivity assumption by estimating propensity scores. Rows in the matrix are levels of covariates that have a zero or near zero probability of treatment. If the matrix is empty, none of the observations have an estimated zero probability of treatment, which implies the positivity assumption is satisfied.

Tip

One can also specify the maxium number of possible treatments to consider for the causal consistency assumption and the minimum and maximum probabilities of treatment for the positivity assumption with the num_treatments, min, and max keyword arguments.

Danger

Obtaining correct estimates is dependent on meeting the assumptions for double machine learning. If the assumptions are not met then any estimates may be biased and lead to incorrect conclusions.

Note

For a thorough review of casual inference assumptions see:

Hernan, Miguel A., and James M. Robins. Causal inference what if. Boca Raton: Taylor and
Francis, 2024.