Address thirty-eighth round of CI review findings on PR #318

P2 methodology (StaggeredTripleDiff fixed-control prose incomplete).
Round-37 moved StaggeredTripleDiff's fixed ``control_group="never_
treated"`` schema to ``n_never_enabled`` (REGISTRY.md line 1730
names the never-enabled cohort as the valid fixed comparison) and
cleared the composite ``n_control_units`` total from ``n_control``.
The renderers, however, only surface ``n_never_enabled`` inside the
``is_dynamic_control`` branch — so the fixed ``never_treated`` path
fell through to the generic ``Sample: N observations.`` sentence
and the full report omitted the fixed comparison cohort entirely.

Added dedicated fixed-never-enabled branches to both renderers:

  * ``_render_summary`` emits ``Sample: N observations (N_t treated,
    N_ne never-enabled).`` when the estimator is
    ``StaggeredTripleDiffResults``, the dynamic branch is not
    active, and ``n_never_enabled > 0``;
  * ``_render_full_report`` emits a dedicated bullet
    ``- Never-enabled units (fixed comparison cohort): N_ne`` under
    the same condition.

P3 coverage. Round-37 regression only asserted absence of the
wrong ``500 control`` wording; it did not positively assert the
valid never-enabled comparison cohort appeared in rendered prose,
which is why the P2 above slipped through. Regressions extended:

  * ``test_never_treated_mode_summary_renders_never_enabled_count``
    asserts ``300 never-enabled`` appears in ``summary()`` AND the
    generic fallback ``Sample: 800 observations.`` does not fire;
  * new ``test_never_treated_mode_full_report_renders_never_enabled_count``
    asserts the sample section of ``full_report()`` names
    ``never-enabled`` and the ``300`` count while omitting any
    bare ``- Control: 500`` line.

278 BR / DR / practitioner tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/business_report.py

@@ -1931,6 +1931,25 @@ def _render_summary(schema: Dict[str, Any]) -> str:
                     "dynamic not-yet-treated comparison group (the control set "
                     f"varies by cohort and period){subset_clause}."
                 )
+        elif (
+            estimator == "StaggeredTripleDiffResults"
+            and isinstance(n_t, int)
+            and isinstance(n_ne, int)
+            and n_ne > 0
+        ):
+            # Round-38 P2 CI review on PR #318: StaggeredTripleDiff
+            # under fixed ``control_group="never_treated"`` had the
+            # schema moved to ``n_never_enabled`` (round-37) but the
+            # renderers fell through to the generic
+            # ``Sample: N observations.`` sentence because the
+            # ``is_dynamic_control`` branch didn't fire. REGISTRY.md
+            # §StaggeredTripleDifference line 1730 names the
+            # never-enabled cohort as the valid fixed comparison on
+            # this path; the prose must say so.
+            sentences.append(
+                f"Sample: {n_obs:,} observations ({n_t:,} treated, "
+                f"{n_ne:,} never-enabled)."
+            )
         else:
             sentences.append(f"Sample: {n_obs:,} observations.")
         survey = sample.get("survey")
@@ -2106,6 +2125,23 @@ def _render_full_report(schema: Dict[str, Any]) -> str:
     )
     if isinstance(sample.get("n_control"), int):
         lines.append(f"- Control: {sample['n_control']:,}")
+    elif (
+        estimator_name == "StaggeredTripleDiffResults"
+        and isinstance(sample.get("n_never_enabled"), int)
+        and sample["n_never_enabled"] > 0
+        and not sample.get("dynamic_control")
+    ):
+        # Round-38 P2 CI review on PR #318: fixed
+        # ``control_group="never_treated"`` on StaggeredTripleDiff
+        # clears ``n_control`` (composite total) and populates
+        # ``n_never_enabled`` (the valid fixed comparison cohort per
+        # REGISTRY.md line 1730). The full report must render that
+        # fixed count — the dynamic-control branch below would not
+        # fire on this path.
+        lines.append(
+            f"- Never-enabled units (fixed comparison cohort): "
+            f"{sample['n_never_enabled']:,}"
+        )
     elif sample.get("dynamic_control"):
         if isinstance(sample.get("n_never_enabled"), int) and sample["n_never_enabled"] > 0:
             lines.append(

tests/test_business_report.py

@@ -1333,18 +1333,38 @@ def test_never_treated_mode_surfaces_never_enabled_not_composite_total(self):
         )
         assert sample["n_never_enabled"] == 300
 
-    def test_never_treated_mode_summary_does_not_narrate_composite_as_control(self):
+    def test_never_treated_mode_summary_renders_never_enabled_count(self):
+        """Round-38 P3 strengthened regression: the summary must
+        POSITIVELY surface the valid fixed comparison cohort
+        (``300 never-enabled``), not merely avoid the wrong
+        ``500 control`` phrasing.
+        """
+        import re
+
         summary = BusinessReport(
             self._stub("never_treated"), auto_diagnostics=False
         ).summary()
-        # The composite total must not appear as "500 control" in prose.
-        import re
-
-        assert not re.search(r"\b500\s+control", summary), (
-            f"BR summary must not narrate the composite n_control_units "
-            f"total as 'control' on StaggeredTripleDiff(control_group="
+        # Old wrong phrasing absent.
+        assert not re.search(r"\b500\s+control", summary), summary
+        # New fixed cohort present.
+        assert "300 never-enabled" in summary, (
+            f"BR summary must render the valid fixed never-enabled "
+            f"comparison cohort on StaggeredTripleDiff(control_group="
             f"'never_treated'); got: {summary!r}"
         )
+        # And the generic no-comparison fallback must not fire.
+        assert "Sample: 800 observations." not in summary
+
+    def test_never_treated_mode_full_report_renders_never_enabled_count(self):
+        md = BusinessReport(
+            self._stub("never_treated"), auto_diagnostics=False
+        ).full_report()
+        sample_section = md.split("## Sample", 1)[1].split("\n## ", 1)[0]
+        assert "never-enabled" in sample_section.lower()
+        assert "300" in sample_section
+        # No bare "- Control: 500" line (composite total) should appear
+        # on this path.
+        assert "- Control: 500" not in sample_section
 
 
 class TestBRHeadlineOmitsBrokenCIOnUndefinedInference: