Address Copilot review: clarify MAP estimate, centralize PSEUDO_COUNT imports

- Clarify Beta(2,2) comment to specify posterior mode (MAP) estimate
- Import PSEUDO_COUNT from repness.py instead of hardcoding in simplified_repness_test.py
- Move PSEUDO_COUNT import to module-level in test_repness_unit.py and test_old_format_repness.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jucor

delphi/polismath/pca_kmeans_rep/repness.py

@@ -24,10 +24,11 @@
 # Why use pseudocounts?
 # - Prevents extreme probabilities (0 or 1) when sample sizes are small
 # - With PSEUDO_COUNT = 2.0, we add 1 "virtual" agree and 1 "virtual" disagree
-#   to each comment's vote count — equivalent to a Beta(2,2) prior
+#   to each comment's vote count — equivalent to using a Beta(2,2) prior and
+#   taking the posterior mode (MAP) estimate
 # - This pulls probabilities toward 0.5, with the effect diminishing as n grows
-# - Formula: p_agree = (n_agree + PSEUDO_COUNT/2) / (n_votes + PSEUDO_COUNT)
-#            i.e.      (n_agree + 1) / (n_votes + 2)
+# - Formula (MAP under Beta(2,2)): p_agree = (n_agree + PSEUDO_COUNT/2) / (n_votes + PSEUDO_COUNT)
+#                                  i.e.      (n_agree + 1) / (n_votes + 2)
 #
 # Matches Clojure's implementation (repness.clj).
 PSEUDO_COUNT = 2.0

delphi/tests/simplified_repness_test.py

@@ -17,7 +17,8 @@
 
 # Constants
 Z_90 = 1.645  # Z-score for 90% confidence
-PSEUDO_COUNT = 2.0  # Pseudocount for Bayesian smoothing (Beta(2,2) prior, matches Clojure)
+
+from polismath.pca_kmeans_rep.repness import PSEUDO_COUNT
 
 def prop_test(p: float, n: int, p0: float) -> float:
     """One-proportion z-test."""

delphi/tests/test_old_format_repness.py

@@ -14,6 +14,7 @@
 sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), '..')))
 
 from polismath.pca_kmeans_rep.repness import (
+    PSEUDO_COUNT,
     z_score_sig_90, z_score_sig_95, prop_test, two_prop_test,
     comment_stats, add_comparative_stats, repness_metric, finalize_cmt_stats,
     passes_by_test, best_agree, best_disagree, select_rep_comments,
@@ -80,7 +81,6 @@ def test_comment_stats(self):
         n_agree = 3
         n_disagree = 1
         n_votes = 4
-        from polismath.pca_kmeans_rep.repness import PSEUDO_COUNT
         p_agree = (n_agree + PSEUDO_COUNT/2) / (n_votes + PSEUDO_COUNT)
         p_disagree = (n_disagree + PSEUDO_COUNT/2) / (n_votes + PSEUDO_COUNT)
 

delphi/tests/test_repness_unit.py

@@ -13,6 +13,7 @@
 sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), '..')))
 
 from polismath.pca_kmeans_rep.repness import (
+    PSEUDO_COUNT,
     z_score_sig_90, z_score_sig_95, prop_test, two_prop_test,
     comment_stats, add_comparative_stats, repness_metric, finalize_cmt_stats,
     passes_by_test, best_agree, best_disagree, select_rep_comments,
@@ -81,7 +82,6 @@ def test_comment_stats(self):
         n_agree = 3
         n_disagree = 1
         n_votes = 4
-        from polismath.pca_kmeans_rep.repness import PSEUDO_COUNT
         p_agree = (n_agree + PSEUDO_COUNT/2) / (n_votes + PSEUDO_COUNT)
         p_disagree = (n_disagree + PSEUDO_COUNT/2) / (n_votes + PSEUDO_COUNT)