Add SyntheticDiD validation diagnostics

BRIEFING.md

@@ -0,0 +1,83 @@
+# SDID Practitioner Validation Tooling - Briefing
+
+## Problem
+
+A data scientist runs `SyntheticDiD`, gets an ATT and a p-value, and then
+faces the question: *should I trust this estimate?* The library gives them the
+point estimate and inference, but the validation workflow - the steps between
+"I got a number" and "I'm confident enough to present this" - is largely
+left to the practitioner to assemble from scratch.
+
+The standard validation workflow for synthetic control methods is well
+understood in the econometrics literature (Arkhangelsky et al. 2021,
+Abadie et al. 2010, Abadie 2021). The pieces include pre-treatment fit
+assessment, weight diagnostics, placebo/falsification tests, sensitivity
+analysis, and cross-estimator comparison. Our library provides some of the
+raw ingredients (pre-treatment RMSE, weight dicts, placebo effects array)
+but doesn't connect them into an accessible diagnostic workflow.
+
+The gap is most visible in `practitioner.py`, where `_handle_synthetic`
+recommends in-time placebos and leave-one-out analysis but provides only
+comment-only pseudo-code. A practitioner following that guidance hits a wall.
+
+## Current state
+
+What we have today:
+
+- `results.pre_treatment_fit` (RMSE) with a warning when it exceeds the
+  treated pre-period SD
+- `results.get_unit_weights_df()` and `results.get_time_weights_df()`
+- Three variance methods: placebo (default), bootstrap, and jackknife (just
+  landed in v3.1.1)
+- `results.placebo_effects` - stores per-iteration estimates for all three
+  variance methods, but for jackknife these are positional LOO estimates
+  with no unit labels
+- `results.summary()` shows top-5 unit weights and count of non-trivial weights
+- `practitioner.py` guidance that names the right steps but can't point to
+  runnable code for most of them
+
+What the practitioner must currently build themselves:
+
+- Mapping jackknife LOO estimates back to unit identities to answer "which
+  unit, when dropped, changes my estimate the most?"
+- In-time placebo tests (re-estimate with a fake treatment date)
+- Any weight concentration metric beyond eyeballing the sorted list
+- Any sense of whether their RMSE is "bad enough to worry about" beyond
+  the binary warning
+- Regularization sensitivity (does the ATT change if I perturb zeta?)
+- Pre-treatment trajectory data for plotting (the Y matrices are internal
+  to `fit()` and not returned)
+
+## Context from prior discussion
+
+The jackknife work created an interesting opportunity. The delete-one-re-estimate
+loop already runs for SE computation. The per-unit ATT estimates are stored in
+`results.placebo_effects`. The missing piece is a presentation layer that maps
+those estimates to unit identities and surfaces the diagnostic interpretation
+(which units are influential, how stable is the estimate to unit composition).
+
+More broadly, the validation gaps fall into two categories:
+
+1. **Low-marginal-cost additions** - things where the computation already
+   exists and we just need to expose or label it (LOO diagnostic from
+   jackknife, weight concentration metrics, trajectory data extraction)
+
+2. **New functionality** - things that require new estimation loops or
+   helpers (in-time placebo, regularization sensitivity sweep)
+
+The practitioner guidance in `practitioner.py` should evolve alongside any
+new tooling so that the recommended steps point to real, runnable code paths.
+
+## What "done" looks like
+
+A practitioner using SyntheticDiD should be able to follow a credible
+validation workflow using library-provided tools and guidance, without
+needing to reverse-engineer internals or write substantial boilerplate.
+The validation steps recognized in the literature should either be directly
+supported or have clear, concrete guidance for how to perform them with
+the library's API.
+
+This is not about adding visualization or plotting (that's a separate
+concern). It's about making the computational and diagnostic building
+blocks accessible and well-documented through the results API and
+practitioner guidance.

diff_diff/practitioner.py

@@ -505,35 +505,74 @@ def _handle_synthetic(results: Any):
     steps = [
         _step(
             baker_step=6,
-            label="Check pre-treatment fit quality",
+            label="Check pre-treatment fit and weight concentration",
             why=(
                 "Synthetic DiD relies on pre-treatment fit to construct "
-                "weights. Poor fit suggests the synthetic control may not "
-                "approximate the counterfactual well."
+                "weights. Poor fit or highly concentrated unit weights "
+                "suggest the synthetic control may not approximate the "
+                "counterfactual well."
             ),
             code=(
-                "# Check pre-treatment fit and unit weight concentration:\n"
                 "print(f'Pre-treatment fit (RMSE): {results.pre_treatment_fit:.4f}')\n"
-                "# Highly concentrated weights suggest fragile estimates"
+                "concentration = results.get_weight_concentration()\n"
+                "print(f\"Effective N: {concentration['effective_n']:.1f}\")\n"
+                "print(f\"Top-5 weight share: {concentration['top_k_share']:.2%}\")"
             ),
             step_name="sensitivity",
         ),
         _step(
             baker_step=6,
-            label="In-time or in-space placebo",
+            label="In-time placebo",
             why=(
-                "Test robustness by re-estimating on a placebo treatment "
-                "period (in-time) or excluding treated units one at a time "
-                "(leave-one-out). These are the natural falsification "
-                "checks for synthetic control methods."
+                "Re-estimate on shifted fake treatment dates in the "
+                "pre-period. A credible design yields near-zero placebo "
+                "ATTs — departures signal that something is being picked "
+                "up pre-treatment, weakening the causal interpretation."
             ),
             code=(
-                "# In-time placebo: re-estimate with a fake treatment date\n"
-                "# Leave-one-out: drop each treated unit and re-estimate"
+                "placebo_df = results.in_time_placebo()\n"
+                "print(placebo_df)"
             ),
             priority="medium",
             step_name="sensitivity",
         ),
+        _step(
+            baker_step=6,
+            label="Leave-one-out influence (jackknife)",
+            why=(
+                "If the estimate is driven by a single unit, robustness "
+                "is weak. Fit with variance_method='jackknife' and inspect "
+                "which units move the ATT the most."
+            ),
+            code=(
+                "# Requires variance_method='jackknife' AND enough support for LOO\n"
+                "# (n_treated >= 2 and >= 2 effective-weight controls).\n"
+                "if getattr(results, '_loo_unit_ids', None) is not None:\n"
+                "    loo_df = results.get_loo_effects_df()\n"
+                "    print(loo_df.head(10))\n"
+                "else:\n"
+                "    print('LOO not available - re-fit with '\n"
+                "          'variance_method=\"jackknife\" and ensure >=2 treated units '\n"
+                "          'with positive effective support.')"
+            ),
+            priority="medium",
+            step_name="sensitivity",
+        ),
+        _step(
+            baker_step=6,
+            label="Regularization sensitivity (zeta_omega)",
+            why=(
+                "The unit-weight regularization is auto-selected from "
+                "data. Show whether the ATT moves materially across a "
+                "grid of values to gauge robustness to this choice."
+            ),
+            code=(
+                "sens_df = results.sensitivity_to_zeta_omega()\n"
+                "print(sens_df)"
+            ),
+            priority="low",
+            step_name="sensitivity",
+        ),
         _step(
             baker_step=8,
             label="Compare with staggered estimators (CS, SA)",