diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0e0e749..87e1b29 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,19 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [2.1.6] - 2026-01-24
+
+### Added
+- **Methodology verification tests** for DifferenceInDifferences estimator
+  - Comprehensive test suite validating all REGISTRY.md requirements
+  - Tests for formula interface, coefficient extraction, rank deficiency handling
+  - Singleton cluster variance estimation behavioral tests
+
+### Changed
+- **REGISTRY.md documentation improvements**
+  - Clarified singleton cluster formula notation (u_i² X_i X_i' instead of ambiguous residual² × X'X)
+  - Verified DifferenceInDifferences behavior against documented requirements
+
 ## [2.1.5] - 2026-01-22
 
 ### Added
@@ -517,6 +530,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - `to_dict()` and `to_dataframe()` export methods
   - `is_significant` and `significance_stars` properties
 
+[2.1.6]: https://github.com/igerber/diff-diff/compare/v2.1.5...v2.1.6
 [2.1.5]: https://github.com/igerber/diff-diff/compare/v2.1.4...v2.1.5
 [2.1.4]: https://github.com/igerber/diff-diff/compare/v2.1.3...v2.1.4
 [2.1.3]: https://github.com/igerber/diff-diff/compare/v2.1.2...v2.1.3
diff --git a/diff_diff/__init__.py b/diff_diff/__init__.py
index 799d9f4..e32c1d7 100644
--- a/diff_diff/__init__.py
+++ b/diff_diff/__init__.py
@@ -136,7 +136,7 @@
     load_mpdta,
 )
 
-__version__ = "2.1.5"
+__version__ = "2.1.6"
 __all__ = [
     # Estimators
     "DifferenceInDifferences",
diff --git a/pyproject.toml b/pyproject.toml
index 0275e78..d76002c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "maturin"
 
 [project]
 name = "diff-diff"
-version = "2.1.5"
+version = "2.1.6"
 description = "A library for Difference-in-Differences causal inference analysis"
 readme = "README.md"
 license = "MIT"
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index 528b8a3..eed07f0 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "diff_diff_rust"
-version = "2.1.5"
+version = "2.1.6"
 edition = "2021"
 description = "Rust backend for diff-diff DiD library"
 license = "MIT"