# Comparing the AMM-side T-learner against External Meta-learners

### Operational recipe: `grf` R-side and EconML Python-side adapters (Sub-phase 8.5.B)

#### José Mauricio Gómez Julián

#### 2026-07-06

# 1. What this vignette covers

This is an operational recipe. It walks you through `gdpar_compare_meta_learners()` end to end with two reference adapters: `gdpar_adapter_grf()` (R-side, based on the `grf` package) and `gdpar_adapter_econml()` (Python-side, based on the `econml` library via `reticulate`). It also shows how to extend the comparator by writing your own adapter, using `DoubleML` as the worked example, and ends with troubleshooting for the Python-side path.

The theoretical canonization of the comparator — definitions, the adapter contract, the concordance criterion, identifiability per arm under cross-method comparison, and the limits of the exercise — lives in the companion canonical vignette `vignette("v08c_meta_learner_comparison")`. Read that one if you want to know why a given choice was made; read this one if you want to do it.

We assume you have a `gdpar_causal_bridge` already built from a pair of `gdpar_fit` objects (one per arm). If you are new to the bridge, see `vignette("v08b_cate_ite_bridge_implementation")` first.

# 2. Setup

Synthetic data, two arms, two fits, one bridge. The example is deliberately small so the chunk runs in a few seconds.

    library(gdpar)
    set.seed(2026L)
    n <- 300L

    df <- data.frame(x1 = rnorm(2L * n))
    df$arm <- rep(c("treat", "ctrl"), each = n)
    df$y <- with(df, ifelse(arm == "treat", 0.5, 0) +
                     0.8 * x1 +
                     rnorm(2L * n, sd = 0.5))
    df_t <- subset(df, arm == "treat"); df_t$arm <- NULL
    df_c <- subset(df, arm == "ctrl");  df_c$arm <- NULL

    fit_t <- gdpar(y ~ x1, amm = amm_spec(a = ~ x1), data = df_t,
                   iter_warmup = 300, iter_sampling = 300, chains = 2)
    fit_c <- gdpar(y ~ x1, amm = amm_spec(a = ~ x1), data = df_c,
                   iter_warmup = 300, iter_sampling = 300, chains = 2)

    newdata <- data.frame(x1 = seq(-2, 2, length.out = 21L))
    bridge <- gdpar_causal_bridge(fit_t, fit_c, newdata = newdata)

From here on we have a `bridge` of class `gdpar_causal_bridge`. The comparator takes that object and a list of adapters and never touches the two fits again.

# 3. The `grf` adapter in three lines

`grf` is in `Suggests`. The adapter is constructed with `gdpar_adapter_grf()`; you only need to pass the hyperparameters you want to override. Sensible defaults match `grf`’s own defaults (`num_trees = 2000L`, honesty on).

    adapter_grf <- gdpar_adapter_grf(num_trees = 500L, seed = 2026L)
    cmp <- gdpar_compare_meta_learners(
      bridge,
      methods = list(grf = adapter_grf)
    )
    print(cmp)
    summary(cmp)

What you should see in `print(cmp)`:

- A line per method (only `grf` here) with `native_ci = TRUE`, the wall-clock time, and `predict = TRUE` (the adapter exposes `predict_fun`, so re-evaluating on a fresh grid does not refit).
- Three small concordance matrices indexed by `bridge` and `grf`: RMSE, Pearson, MAD.

If you want to re-evaluate the comparison on a fresh grid without refitting `grf`:

    newdata2 <- data.frame(x1 = seq(-1.5, 1.5, length.out = 15L))
    cmp_new  <- predict(cmp, newdata = newdata2)

`predict.gdpar_meta_learner_comparison` reuses the cached `state` inside `cmp$external$grf$state` (the fitted `grf::causal_forest` object). The bridge is re-evaluated on `newdata2` via the embedded `gdpar_causal_bridge` and the new concordance matrices are recomputed.

# 4. The EconML adapter

`reticulate` and the Python module `econml` are both optional. `reticulate` is in `Suggests`; `econml` is a Python package that lives in your active Python environment. The package does not install Python dependencies on your behalf.

## 4.1. One-time installation

The recommended flow:

    # 1. Install reticulate (R-side) if absent.
    install.packages("reticulate")

    # 2. Register econml as a Python requirement, then install it.
    reticulate::py_require("econml")  # reticulate 1.46+ ephemeral-env style
    reticulate::py_install("econml")  # adds econml to the active env

    # 3. Verify.
    reticulate::py_module_available("econml")  # should be TRUE

The `py_require` call is a no-op on reticulate releases that predate the ephemeral-env management; on 1.46 and later it tells reticulate which Python module to pin in the active uv-managed environment. `py_install` then performs the actual installation when needed.

If you maintain a virtualenv or conda env explicitly, install `econml` in it with `pip install econml` or `conda install -c conda-forge econml` and point `reticulate` to it via `RETICULATE_PYTHON` or `reticulate::use_virtualenv()` / `use_condaenv()`.

## 4.2. Running the EconML adapter

Once the Python module is available, the adapter is constructed exactly like the `grf` one:

    adapter_econml <- gdpar_adapter_econml(n_estimators = 500L, seed = 2026L)
    cmp2 <- gdpar_compare_meta_learners(
      bridge,
      methods = list(grf    = adapter_grf,
                     econml = adapter_econml)
    )
    print(cmp2)
    summary(cmp2)

The concordance matrices now have three rows / columns (`bridge`, `grf`, `econml`). Both adapters expose `predict_fun`, so `predict(cmp2, newdata = newdata_fresh)` reuses both cached states without refitting either model.

## 4.3. Caveat: serialization of the EconML state

The `state` slot for the EconML adapter holds a reference to a Python object managed by `reticulate`. The reference is valid for the duration of the R session in which the comparison was built; `saveRDS(cmp2, file = ...)` and a fresh-session `readRDS` will lose that reference, and a subsequent call to `predict(cmp2_restored, ...)` aborts cleanly with `gdpar_unsupported_feature_error`. Either re-fit in the new session or build the comparison there from scratch. The `grf` state survives serialization without modification.

# 5. Reading the output

The print method emits four blocks:

    <gdpar_meta_learner_comparison>
      n_obs                : <number of evaluation rows>
      n_methods (external) : <length of methods>
      level                : <inherited from bridge>
      methods              :
        - grf            native_ci = TRUE  time = ...  notes = 0  predict = TRUE
        - econml         native_ci = TRUE  time = ...  notes = 0  predict = TRUE

      concordance matrices (m-by-m, m = 1 + n_methods):
        RMSE: <symmetric m-by-m matrix>
        Pearson: <symmetric m-by-m matrix>
        MAD: <symmetric m-by-m matrix>

- A small RMSE between `bridge` and an external method indicates point-estimate agreement on `cate_mean` across the evaluation grid.
- A high Pearson with small RMSE means the two methods agree both in shape and in absolute scale. A high Pearson with large RMSE means the two methods agree on the shape of the CATE surface but disagree on the level (e.g. one is shifted).
- MAD is more robust than RMSE in the presence of a few outliers.

The summary method (`summary(cmp)`) returns a structured object with three slots: `ate_table` (one row per method with the marginal ATE and CI bounds), `metrics` (the long-format version of the three matrices), `timing` (a per-method timing table). Use it for tables in reports.

# 6. Writing your own adapter (DoubleML as an example)

The contract of an adapter is two functions:

- `fit_predict_fun(X, Y, T, X_newdata, level, seed_run)` returns `list(cate_mean, cate_ci, state, notes)`.
- `predict_fun(state, X_newdata, level)` (optional) returns `list(cate_mean, cate_ci)`. When absent, `predict()` falls back to `fit_predict_fun` and emits a `gdpar_diagnostic_warning`.

A worked sketch with `DoubleML` (R-side; install with `install.packages("DoubleML")`):

    fit_predict_dml <- function(X, Y, T, X_newdata, level, seed_run) {
      if (!requireNamespace("DoubleML", quietly = TRUE) ||
          !requireNamespace("mlr3learners", quietly = TRUE)) {
        stop("DoubleML and mlr3learners are required for this adapter.")
      }
      d <- cbind(X, Y = as.numeric(Y), T = as.integer(T))
      dml_data <- DoubleML::DoubleMLData$new(d, y_col = "Y", d_cols = "T",
                                              x_cols = setdiff(colnames(d),
                                                                c("Y", "T")))
      learner_g <- mlr3::lrn("regr.ranger", num.trees = 200L)
      learner_m <- mlr3::lrn("classif.ranger", num.trees = 200L,
                              predict_type = "prob")
      model <- DoubleML::DoubleMLPLR$new(dml_data, ml_g = learner_g$clone(),
                                          ml_m = learner_m$clone())
      model$fit()
      est <- as.numeric(model$coef)
      est_se <- as.numeric(model$se)
      z <- stats::qnorm(1 - (1 - level) / 2)
      n_new <- nrow(X_newdata)
      list(
        cate_mean = rep(est, n_new),
        cate_ci   = cbind(lower = rep(est - z * est_se, n_new),
                           upper = rep(est + z * est_se, n_new)),
        state     = list(model = model),
        notes     = "DoubleMLPLR returns a single ATE coefficient; broadcast to a constant CATE."
      )
    }

    predict_dml <- function(state, X_newdata, level) {
      n_new <- nrow(X_newdata)
      est <- as.numeric(state$model$coef)
      est_se <- as.numeric(state$model$se)
      z <- stats::qnorm(1 - (1 - level) / 2)
      list(
        cate_mean = rep(est, n_new),
        cate_ci   = cbind(lower = rep(est - z * est_se, n_new),
                           upper = rep(est + z * est_se, n_new))
      )
    }

    adapter_dml <- gdpar_meta_learner_adapter(
      name = "doubleml_plr",
      fit_predict_fun = fit_predict_dml,
      predict_fun = predict_dml,
      requires_r = c("DoubleML", "mlr3", "mlr3learners"),
      native_ci = TRUE,
      description = "DoubleMLPLR (constant CATE; useful as a robust ATE benchmark)"
    )

    cmp_with_dml <- gdpar_compare_meta_learners(
      bridge,
      methods = list(grf = adapter_grf, dml = adapter_dml)
    )

A few notes on this sketch:

- `DoubleMLPLR` returns a single coefficient (an ATE), so the `cate_mean` is broadcast as a constant vector. That is honest: the partially linear model does not estimate a heterogeneous effect.
- If you want heterogeneity from `DoubleML`, use `DoubleMLIRM` with an interactive design and post-process the effect surface as you see fit; the contract is the same.
- The `notes` slot is the right place to document such quirks; the comparator surfaces it through `print()` and `summary()`.

# 7. Troubleshooting (Python-side)

A short catalogue of what tends to go wrong and what to do.

(a) `reticulate` not installed. Install it with `install.packages("reticulate")`. The package itself is small; the heavy lifting is on the Python side.

(b) `econml` not available. Run `reticulate::py_install("econml")`. On reticulate 1.46 and later, reticulate uses ephemeral environments managed by `uv`; you may need to call `reticulate::py_require("econml")` to pin the package in the active environment before any Python operation.

(c) Wrong Python detected. Inspect `reticulate::py_config()` and override with `Sys.setenv(RETICULATE_PYTHON = "/path/to/python")` before loading reticulate, or use the explicit `reticulate::use_virtualenv(...)`. If you maintain a Conda environment, `reticulate::use_condaenv("name")` does the same.

(d) `numpy` complains about version conflicts. Pin the numpy version your `econml` was built against: `reticulate::py_install("numpy==1.26.*", pip = TRUE)`. EconML 0.16 is known to work with numpy 1.26.x as of mid-2026.

(e) Cached Python state lost after restart. The Python objects inside `cmp$external$econml$state` do not survive R session restarts. Rebuild the comparison in the new session.

(f) Adapter aborts with `gdpar_missing_dependency_error`. That error is the package’s deliberate, structured signal that a `Suggests` package or a Python module is missing. The error message names the missing item; install it and rerun. The package never installs anything on your behalf.

# 8. Where to go next

- `vignette("v08c_meta_learner_comparison")` — the canonical theoretical addendum (definitions, identification under cross-method comparison, the concordance criterion, the limits of the exercise, identifiability per arm under the bridge).
- `vignette("v08b_cate_ite_bridge_implementation")` — the canonical T-learner AMM-side bridge (the object you feed into the comparator).
- `vignette("v08_cate_ite_positioning")` — the positioning of the package’s CATE / ITE workflow within the meta-learner literature.

End of Operational Vignette – Sub-phase 8.5.B.