diff --git a/gtsam/discrete/AlgebraicDecisionTree.h b/gtsam/discrete/AlgebraicDecisionTree.h
index 25e18cfaf2..b3f0d69b0e 100644
--- a/gtsam/discrete/AlgebraicDecisionTree.h
+++ b/gtsam/discrete/AlgebraicDecisionTree.h
@@ -71,7 +71,7 @@ namespace gtsam {
       static inline double id(const double& x) { return x; }
     };
 
-    AlgebraicDecisionTree() : Base(1.0) {}
+    AlgebraicDecisionTree(double leaf = 1.0) : Base(leaf) {}
 
     // Explicitly non-explicit constructor
     AlgebraicDecisionTree(const Base& add) : Base(add) {}
@@ -158,9 +158,9 @@ namespace gtsam {
     }
 
     /// print method customized to value type `double`.
-    void print(const std::string& s,
-              const typename Base::LabelFormatter& labelFormatter =
-                  &DefaultFormatter) const {
+    void print(const std::string& s = "",
+               const typename Base::LabelFormatter& labelFormatter =
+                   &DefaultFormatter) const {
       auto valueFormatter = [](const double& v) {
         return (boost::format("%4.8g") % v).str();
       };
diff --git a/gtsam/hybrid/GaussianMixture.cpp b/gtsam/hybrid/GaussianMixture.cpp
index fa0cc189b6..c0815b2d7a 100644
--- a/gtsam/hybrid/GaussianMixture.cpp
+++ b/gtsam/hybrid/GaussianMixture.cpp
@@ -85,8 +85,8 @@ size_t GaussianMixture::nrComponents() const {
 
 /* *******************************************************************************/
 GaussianConditional::shared_ptr GaussianMixture::operator()(
-    const DiscreteValues &discreteVals) const {
-  auto &ptr = conditionals_(discreteVals);
+    const DiscreteValues &discreteValues) const {
+  auto &ptr = conditionals_(discreteValues);
   if (!ptr) return nullptr;
   auto conditional = boost::dynamic_pointer_cast<GaussianConditional>(ptr);
   if (conditional)
@@ -207,4 +207,30 @@ void GaussianMixture::prune(const DecisionTreeFactor &decisionTree) {
   conditionals_.root_ = pruned_conditionals.root_;
 }
 
+/* *******************************************************************************/
+AlgebraicDecisionTree<Key> GaussianMixture::error(
+    const VectorValues &continuousValues) const {
+  // functor to calculate to double error value from GaussianConditional.
+  auto errorFunc =
+      [continuousValues](const GaussianConditional::shared_ptr &conditional) {
+        if (conditional) {
+          return conditional->error(continuousValues);
+        } else {
+          // Return arbitrarily large error if conditional is null
+          // Conditional is null if it is pruned out.
+          return 1e50;
+        }
+      };
+  DecisionTree<Key, double> errorTree(conditionals_, errorFunc);
+  return errorTree;
+}
+
+/* *******************************************************************************/
+double GaussianMixture::error(const VectorValues &continuousValues,
+                              const DiscreteValues &discreteValues) const {
+  // Directly index to get the conditional, no need to build the whole tree.
+  auto conditional = conditionals_(discreteValues);
+  return conditional->error(continuousValues);
+}
+
 }  // namespace gtsam
diff --git a/gtsam/hybrid/GaussianMixture.h b/gtsam/hybrid/GaussianMixture.h
index dada35ae4b..88d5a02c0c 100644
--- a/gtsam/hybrid/GaussianMixture.h
+++ b/gtsam/hybrid/GaussianMixture.h
@@ -122,7 +122,7 @@ class GTSAM_EXPORT GaussianMixture
   /// @{
 
   GaussianConditional::shared_ptr operator()(
-      const DiscreteValues &discreteVals) const;
+      const DiscreteValues &discreteValues) const;
 
   /// Returns the total number of continuous components
   size_t nrComponents() const;
@@ -144,6 +144,26 @@ class GTSAM_EXPORT GaussianMixture
   /// Getter for the underlying Conditionals DecisionTree
   const Conditionals &conditionals();
 
+  /**
+   * @brief Compute error of the GaussianMixture as a tree.
+   *
+   * @param continuousValues The continuous VectorValues.
+   * @return AlgebraicDecisionTree<Key> A decision tree with the same keys
+   * as the conditionals, and leaf values as the error.
+   */
+  AlgebraicDecisionTree<Key> error(const VectorValues &continuousValues) const;
+
+  /**
+   * @brief Compute the error of this Gaussian Mixture given the continuous
+   * values and a discrete assignment.
+   *
+   * @param continuousValues Continuous values at which to compute the error.
+   * @param discreteValues The discrete assignment for a specific mode sequence.
+   * @return double
+   */
+  double error(const VectorValues &continuousValues,
+               const DiscreteValues &discreteValues) const;
+
   /**
    * @brief Prune the decision tree of Gaussian factors as per the discrete
    * `decisionTree`.
diff --git a/gtsam/hybrid/GaussianMixtureFactor.cpp b/gtsam/hybrid/GaussianMixtureFactor.cpp
index 181b1e6a5a..fd437f52c0 100644
--- a/gtsam/hybrid/GaussianMixtureFactor.cpp
+++ b/gtsam/hybrid/GaussianMixtureFactor.cpp
@@ -95,4 +95,26 @@ GaussianMixtureFactor::Sum GaussianMixtureFactor::asGaussianFactorGraphTree()
   };
   return {factors_, wrap};
 }
+
+/* *******************************************************************************/
+AlgebraicDecisionTree<Key> GaussianMixtureFactor::error(
+    const VectorValues &continuousValues) const {
+  // functor to convert from sharedFactor to double error value.
+  auto errorFunc =
+      [continuousValues](const GaussianFactor::shared_ptr &factor) {
+        return factor->error(continuousValues);
+      };
+  DecisionTree<Key, double> errorTree(factors_, errorFunc);
+  return errorTree;
+}
+
+/* *******************************************************************************/
+double GaussianMixtureFactor::error(
+    const VectorValues &continuousValues,
+    const DiscreteValues &discreteValues) const {
+  // Directly index to get the conditional, no need to build the whole tree.
+  auto factor = factors_(discreteValues);
+  return factor->error(continuousValues);
+}
+
 }  // namespace gtsam
diff --git a/gtsam/hybrid/GaussianMixtureFactor.h b/gtsam/hybrid/GaussianMixtureFactor.h
index 0d649f8576..0b65b5aa93 100644
--- a/gtsam/hybrid/GaussianMixtureFactor.h
+++ b/gtsam/hybrid/GaussianMixtureFactor.h
@@ -20,15 +20,19 @@
 
 #pragma once
 
+#include <gtsam/discrete/AlgebraicDecisionTree.h>
 #include <gtsam/discrete/DecisionTree.h>
 #include <gtsam/discrete/DiscreteKey.h>
+#include <gtsam/discrete/DiscreteValues.h>
 #include <gtsam/hybrid/HybridGaussianFactor.h>
 #include <gtsam/linear/GaussianFactor.h>
+#include <gtsam/linear/VectorValues.h>
 
 namespace gtsam {
 
 class GaussianFactorGraph;
 
+// Needed for wrapper.
 using GaussianFactorVector = std::vector<gtsam::GaussianFactor::shared_ptr>;
 
 /**
@@ -126,6 +130,26 @@ class GTSAM_EXPORT GaussianMixtureFactor : public HybridFactor {
    */
   Sum add(const Sum &sum) const;
 
+  /**
+   * @brief Compute error of the GaussianMixtureFactor as a tree.
+   *
+   * @param continuousValues The continuous VectorValues.
+   * @return AlgebraicDecisionTree<Key> A decision tree with the same keys
+   * as the factors involved, and leaf values as the error.
+   */
+  AlgebraicDecisionTree<Key> error(const VectorValues &continuousValues) const;
+
+  /**
+   * @brief Compute the error of this Gaussian Mixture given the continuous
+   * values and a discrete assignment.
+   *
+   * @param continuousValues Continuous values at which to compute the error.
+   * @param discreteValues The discrete assignment for a specific mode sequence.
+   * @return double
+   */
+  double error(const VectorValues &continuousValues,
+               const DiscreteValues &discreteValues) const;
+
   /// Add MixtureFactor to a Sum, syntactic sugar.
   friend Sum &operator+=(Sum &sum, const GaussianMixtureFactor &factor) {
     sum = factor.add(sum);
diff --git a/gtsam/hybrid/HybridBayesNet.cpp b/gtsam/hybrid/HybridBayesNet.cpp
index a641363846..48c4b6d508 100644
--- a/gtsam/hybrid/HybridBayesNet.cpp
+++ b/gtsam/hybrid/HybridBayesNet.cpp
@@ -232,4 +232,56 @@ VectorValues HybridBayesNet::optimize(const DiscreteValues &assignment) const {
   return gbn.optimize();
 }
 
+/* ************************************************************************* */
+double HybridBayesNet::error(const VectorValues &continuousValues,
+                             const DiscreteValues &discreteValues) const {
+  GaussianBayesNet gbn = this->choose(discreteValues);
+  return gbn.error(continuousValues);
+}
+
+/* ************************************************************************* */
+AlgebraicDecisionTree<Key> HybridBayesNet::error(
+    const VectorValues &continuousValues) const {
+  AlgebraicDecisionTree<Key> error_tree;
+
+  // Iterate over each factor.
+  for (size_t idx = 0; idx < size(); idx++) {
+    AlgebraicDecisionTree<Key> conditional_error;
+
+    if (factors_.at(idx)->isHybrid()) {
+      // If factor is hybrid, select based on assignment and compute error.
+      GaussianMixture::shared_ptr gm = this->atMixture(idx);
+      conditional_error = gm->error(continuousValues);
+
+      // Assign for the first index, add error for subsequent ones.
+      if (idx == 0) {
+        error_tree = conditional_error;
+      } else {
+        error_tree = error_tree + conditional_error;
+      }
+
+    } else if (factors_.at(idx)->isContinuous()) {
+      // If continuous only, get the (double) error
+      // and add it to the error_tree
+      double error = this->atGaussian(idx)->error(continuousValues);
+      // Add the computed error to every leaf of the error tree.
+      error_tree = error_tree.apply(
+          [error](double leaf_value) { return leaf_value + error; });
+
+    } else if (factors_.at(idx)->isDiscrete()) {
+      // If factor at `idx` is discrete-only, we skip.
+      continue;
+    }
+  }
+
+  return error_tree;
+}
+
+/* ************************************************************************* */
+AlgebraicDecisionTree<Key> HybridBayesNet::probPrime(
+    const VectorValues &continuousValues) const {
+  AlgebraicDecisionTree<Key> error_tree = this->error(continuousValues);
+  return error_tree.apply([](double error) { return exp(-error); });
+}
+
 }  // namespace gtsam
diff --git a/gtsam/hybrid/HybridBayesNet.h b/gtsam/hybrid/HybridBayesNet.h
index 84c1da6ea8..f8ec609119 100644
--- a/gtsam/hybrid/HybridBayesNet.h
+++ b/gtsam/hybrid/HybridBayesNet.h
@@ -124,6 +124,39 @@ class GTSAM_EXPORT HybridBayesNet : public BayesNet<HybridConditional> {
   /// Prune the Hybrid Bayes Net such that we have at most maxNrLeaves leaves.
   HybridBayesNet prune(size_t maxNrLeaves);
 
+  /**
+   * @brief 0.5 * sum of squared Mahalanobis distances
+   * for a specific discrete assignment.
+   *
+   * @param continuousValues Continuous values at which to compute the error.
+   * @param discreteValues Discrete assignment for a specific mode sequence.
+   * @return double
+   */
+  double error(const VectorValues &continuousValues,
+               const DiscreteValues &discreteValues) const;
+
+  /**
+   * @brief Compute conditional error for each discrete assignment,
+   * and return as a tree.
+   *
+   * @param continuousValues Continuous values at which to compute the error.
+   * @return AlgebraicDecisionTree<Key>
+   */
+  AlgebraicDecisionTree<Key> error(const VectorValues &continuousValues) const;
+
+  /**
+   * @brief Compute unnormalized probability q(μ|M),
+   * for each discrete assignment, and return as a tree.
+   * q(μ|M) is the unnormalized probability at the MLE point μ,
+   * conditioned on the discrete variables.
+   *
+   * @param continuousValues Continuous values at which to compute the
+   * probability.
+   * @return AlgebraicDecisionTree<Key>
+   */
+  AlgebraicDecisionTree<Key> probPrime(
+      const VectorValues &continuousValues) const;
+
   /// @}
 
  private:
diff --git a/gtsam/hybrid/HybridGaussianFactorGraph.cpp b/gtsam/hybrid/HybridGaussianFactorGraph.cpp
index 041603fbd5..32653bdecf 100644
--- a/gtsam/hybrid/HybridGaussianFactorGraph.cpp
+++ b/gtsam/hybrid/HybridGaussianFactorGraph.cpp
@@ -423,4 +423,58 @@ const Ordering HybridGaussianFactorGraph::getHybridOrdering() const {
   return ordering;
 }
 
+/* ************************************************************************ */
+AlgebraicDecisionTree<Key> HybridGaussianFactorGraph::error(
+    const VectorValues &continuousValues) const {
+  AlgebraicDecisionTree<Key> error_tree(0.0);
+
+  // Iterate over each factor.
+  for (size_t idx = 0; idx < size(); idx++) {
+    AlgebraicDecisionTree<Key> factor_error;
+
+    if (factors_.at(idx)->isHybrid()) {
+      // If factor is hybrid, select based on assignment.
+      GaussianMixtureFactor::shared_ptr gaussianMixture =
+          boost::static_pointer_cast<GaussianMixtureFactor>(factors_.at(idx));
+      // Compute factor error.
+      factor_error = gaussianMixture->error(continuousValues);
+
+      // If first factor, assign error, else add it.
+      if (idx == 0) {
+        error_tree = factor_error;
+      } else {
+        error_tree = error_tree + factor_error;
+      }
+
+    } else if (factors_.at(idx)->isContinuous()) {
+      // If continuous only, get the (double) error
+      // and add it to the error_tree
+      auto hybridGaussianFactor =
+          boost::static_pointer_cast<HybridGaussianFactor>(factors_.at(idx));
+      GaussianFactor::shared_ptr gaussian = hybridGaussianFactor->inner();
+
+      // Compute the error of the gaussian factor.
+      double error = gaussian->error(continuousValues);
+      // Add the gaussian factor error to every leaf of the error tree.
+      error_tree = error_tree.apply(
+          [error](double leaf_value) { return leaf_value + error; });
+
+    } else if (factors_.at(idx)->isDiscrete()) {
+      // If factor at `idx` is discrete-only, we skip.
+      continue;
+    }
+  }
+
+  return error_tree;
+}
+
+/* ************************************************************************ */
+AlgebraicDecisionTree<Key> HybridGaussianFactorGraph::probPrime(
+    const VectorValues &continuousValues) const {
+  AlgebraicDecisionTree<Key> error_tree = this->error(continuousValues);
+  AlgebraicDecisionTree<Key> prob_tree =
+      error_tree.apply([](double error) { return exp(-error); });
+  return prob_tree;
+}
+
 }  // namespace gtsam
diff --git a/gtsam/hybrid/HybridGaussianFactorGraph.h b/gtsam/hybrid/HybridGaussianFactorGraph.h
index 6a03625001..ac9ae1a462 100644
--- a/gtsam/hybrid/HybridGaussianFactorGraph.h
+++ b/gtsam/hybrid/HybridGaussianFactorGraph.h
@@ -41,7 +41,7 @@ class JacobianFactor;
 
 /**
  * @brief Main elimination function for HybridGaussianFactorGraph.
- * 
+ *
  * @param factors The factor graph to eliminate.
  * @param keys The elimination ordering.
  * @return The conditional on the ordering keys and the remaining factors.
@@ -99,11 +99,12 @@ class GTSAM_EXPORT HybridGaussianFactorGraph
   using shared_ptr = boost::shared_ptr<This>;  ///< shared_ptr to This
 
   using Values = gtsam::Values;  ///< backwards compatibility
-  using Indices = KeyVector;     ///> map from keys to values
+  using Indices = KeyVector;     ///< map from keys to values
 
   /// @name Constructors
   /// @{
 
+  /// @brief Default constructor.
   HybridGaussianFactorGraph() = default;
 
   /**
@@ -170,6 +171,28 @@ class GTSAM_EXPORT HybridGaussianFactorGraph
     }
   }
 
+  /**
+   * @brief Compute error for each discrete assignment,
+   * and return as a tree.
+   *
+   * Error \f$ e = \Vert x - \mu \Vert_{\Sigma} \f$.
+   *
+   * @param continuousValues Continuous values at which to compute the error.
+   * @return AlgebraicDecisionTree<Key>
+   */
+  AlgebraicDecisionTree<Key> error(const VectorValues& continuousValues) const;
+
+  /**
+   * @brief Compute unnormalized probability \f$ P(X | M, Z) \f$
+   * for each discrete assignment, and return as a tree.
+   *
+   * @param continuousValues Continuous values at which to compute the
+   * probability.
+   * @return AlgebraicDecisionTree<Key>
+   */
+  AlgebraicDecisionTree<Key> probPrime(
+      const VectorValues& continuousValues) const;
+
   /**
    * @brief Return a Colamd constrained ordering where the discrete keys are
    * eliminated after the continuous keys.
diff --git a/gtsam/hybrid/MixtureFactor.h b/gtsam/hybrid/MixtureFactor.h
index 5e7337d0cc..f29a840229 100644
--- a/gtsam/hybrid/MixtureFactor.h
+++ b/gtsam/hybrid/MixtureFactor.h
@@ -23,6 +23,7 @@
 #include <gtsam/hybrid/GaussianMixtureFactor.h>
 #include <gtsam/hybrid/HybridNonlinearFactor.h>
 #include <gtsam/nonlinear/NonlinearFactor.h>
+#include <gtsam/nonlinear/NonlinearFactorGraph.h>
 #include <gtsam/nonlinear/Symbol.h>
 
 #include <algorithm>
@@ -86,11 +87,11 @@ class MixtureFactor : public HybridFactor {
    * elements based on the number of discrete keys and the cardinality of the
    * keys, so that the decision tree is constructed appropriately.
    *
-   * @tparam FACTOR The type of the factor shared pointers being passed in. Will
-   * be typecast to NonlinearFactor shared pointers.
+   * @tparam FACTOR The type of the factor shared pointers being passed in.
+   * Will be typecast to NonlinearFactor shared pointers.
    * @param keys Vector of keys for continuous factors.
    * @param discreteKeys Vector of discrete keys.
-   * @param factors Vector of shared pointers to factors.
+   * @param factors Vector of nonlinear factors.
    * @param normalized Flag indicating if the factor error is already
    * normalized.
    */
@@ -107,8 +108,12 @@ class MixtureFactor : public HybridFactor {
       std::copy(f->keys().begin(), f->keys().end(),
                 std::inserter(factor_keys_set, factor_keys_set.end()));
 
-      nonlinear_factors.push_back(
-          boost::dynamic_pointer_cast<NonlinearFactor>(f));
+      if (auto nf = boost::dynamic_pointer_cast<NonlinearFactor>(f)) {
+        nonlinear_factors.push_back(nf);
+      } else {
+        throw std::runtime_error(
+            "Factors passed into MixtureFactor need to be nonlinear!");
+      }
     }
     factors_ = Factors(discreteKeys, nonlinear_factors);
 
@@ -121,22 +126,39 @@ class MixtureFactor : public HybridFactor {
 
   ~MixtureFactor() = default;
 
+  /**
+   * @brief Compute error of the MixtureFactor as a tree.
+   *
+   * @param continuousValues The continuous values for which to compute the
+   * error.
+   * @return AlgebraicDecisionTree<Key> A decision tree with the same keys
+   * as the factor, and leaf values as the error.
+   */
+  AlgebraicDecisionTree<Key> error(const Values& continuousValues) const {
+    // functor to convert from sharedFactor to double error value.
+    auto errorFunc = [continuousValues](const sharedFactor& factor) {
+      return factor->error(continuousValues);
+    };
+    DecisionTree<Key, double> errorTree(factors_, errorFunc);
+    return errorTree;
+  }
+
   /**
    * @brief Compute error of factor given both continuous and discrete values.
    *
-   * @param continuousVals The continuous Values.
-   * @param discreteVals The discrete Values.
+   * @param continuousValues The continuous Values.
+   * @param discreteValues The discrete Values.
    * @return double The error of this factor.
    */
-  double error(const Values& continuousVals,
-               const DiscreteValues& discreteVals) const {
-    // Retrieve the factor corresponding to the assignment in discreteVals.
-    auto factor = factors_(discreteVals);
+  double error(const Values& continuousValues,
+               const DiscreteValues& discreteValues) const {
+    // Retrieve the factor corresponding to the assignment in discreteValues.
+    auto factor = factors_(discreteValues);
     // Compute the error for the selected factor
-    const double factorError = factor->error(continuousVals);
+    const double factorError = factor->error(continuousValues);
     if (normalized_) return factorError;
-    return factorError +
-           this->nonlinearFactorLogNormalizingConstant(factor, continuousVals);
+    return factorError + this->nonlinearFactorLogNormalizingConstant(
+                             factor, continuousValues);
   }
 
   size_t dim() const {
@@ -149,7 +171,7 @@ class MixtureFactor : public HybridFactor {
 
   /// print to stdout
   void print(
-      const std::string& s = "MixtureFactor",
+      const std::string& s = "",
       const KeyFormatter& keyFormatter = DefaultKeyFormatter) const override {
     std::cout << (s.empty() ? "" : s + " ");
     Base::print("", keyFormatter);
@@ -192,17 +214,18 @@ class MixtureFactor : public HybridFactor {
   /// Linearize specific nonlinear factors based on the assignment in
   /// discreteValues.
   GaussianFactor::shared_ptr linearize(
-      const Values& continuousVals, const DiscreteValues& discreteVals) const {
-    auto factor = factors_(discreteVals);
-    return factor->linearize(continuousVals);
+      const Values& continuousValues,
+      const DiscreteValues& discreteValues) const {
+    auto factor = factors_(discreteValues);
+    return factor->linearize(continuousValues);
   }
 
   /// Linearize all the continuous factors to get a GaussianMixtureFactor.
   boost::shared_ptr<GaussianMixtureFactor> linearize(
-      const Values& continuousVals) const {
+      const Values& continuousValues) const {
     // functional to linearize each factor in the decision tree
-    auto linearizeDT = [continuousVals](const sharedFactor& factor) {
-      return factor->linearize(continuousVals);
+    auto linearizeDT = [continuousValues](const sharedFactor& factor) {
+      return factor->linearize(continuousValues);
     };
 
     DecisionTree<Key, GaussianFactor::shared_ptr> linearized_factors(
diff --git a/gtsam/hybrid/hybrid.i b/gtsam/hybrid/hybrid.i
index 86029a48aa..899c129e00 100644
--- a/gtsam/hybrid/hybrid.i
+++ b/gtsam/hybrid/hybrid.i
@@ -196,22 +196,24 @@ class HybridNonlinearFactorGraph {
 
 #include <gtsam/hybrid/MixtureFactor.h>
 class MixtureFactor : gtsam::HybridFactor {
-  MixtureFactor(const gtsam::KeyVector& keys, const gtsam::DiscreteKeys& discreteKeys,
-                const gtsam::DecisionTree<gtsam::Key, gtsam::NonlinearFactor*>& factors, bool normalized = false);
+  MixtureFactor(
+      const gtsam::KeyVector& keys, const gtsam::DiscreteKeys& discreteKeys,
+      const gtsam::DecisionTree<gtsam::Key, gtsam::NonlinearFactor*>& factors,
+      bool normalized = false);
 
   template <FACTOR = {gtsam::NonlinearFactor}>
   MixtureFactor(const gtsam::KeyVector& keys, const gtsam::DiscreteKeys& discreteKeys,
                 const std::vector<FACTOR*>& factors,
                 bool normalized = false);
 
-  double error(const gtsam::Values& continuousVals,
-               const gtsam::DiscreteValues& discreteVals) const;
+  double error(const gtsam::Values& continuousValues,
+               const gtsam::DiscreteValues& discreteValues) const;
 
   double nonlinearFactorLogNormalizingConstant(const gtsam::NonlinearFactor* factor,
                                                const gtsam::Values& values) const;
 
   GaussianMixtureFactor* linearize(
-      const gtsam::Values& continuousVals) const;
+      const gtsam::Values& continuousValues) const;
 
   void print(string s = "MixtureFactor\n",
              const gtsam::KeyFormatter& keyFormatter =
diff --git a/gtsam/hybrid/tests/testGaussianMixture.cpp b/gtsam/hybrid/tests/testGaussianMixture.cpp
index 420e22315b..310081f028 100644
--- a/gtsam/hybrid/tests/testGaussianMixture.cpp
+++ b/gtsam/hybrid/tests/testGaussianMixture.cpp
@@ -78,15 +78,58 @@ TEST(GaussianMixture, Equals) {
   GaussianMixture::Conditionals conditionals(
       {m1},
       vector<GaussianConditional::shared_ptr>{conditional0, conditional1});
-  GaussianMixture mixtureFactor({X(1)}, {X(2)}, {m1}, conditionals);
+  GaussianMixture mixture({X(1)}, {X(2)}, {m1}, conditionals);
 
   // Let's check that this worked:
   DiscreteValues mode;
   mode[m1.first] = 1;
-  auto actual = mixtureFactor(mode);
+  auto actual = mixture(mode);
   EXPECT(actual == conditional1);
 }
 
+/* ************************************************************************* */
+/// Test error method of GaussianMixture.
+TEST(GaussianMixture, Error) {
+  Matrix22 S1 = Matrix22::Identity();
+  Matrix22 S2 = Matrix22::Identity() * 2;
+  Matrix22 R1 = Matrix22::Ones();
+  Matrix22 R2 = Matrix22::Ones();
+  Vector2 d1(1, 2), d2(2, 1);
+
+  SharedDiagonal model = noiseModel::Diagonal::Sigmas(Vector2(1.0, 0.34));
+
+  auto conditional0 = boost::make_shared<GaussianConditional>(X(1), d1, R1,
+                                                              X(2), S1, model),
+       conditional1 = boost::make_shared<GaussianConditional>(X(1), d2, R2,
+                                                              X(2), S2, model);
+
+  // Create decision tree
+  DiscreteKey m1(M(1), 2);
+  GaussianMixture::Conditionals conditionals(
+      {m1},
+      vector<GaussianConditional::shared_ptr>{conditional0, conditional1});
+  GaussianMixture mixture({X(1)}, {X(2)}, {m1}, conditionals);
+
+  VectorValues values;
+  values.insert(X(1), Vector2::Ones());
+  values.insert(X(2), Vector2::Zero());
+  auto error_tree = mixture.error(values);
+
+  // regression
+  std::vector<DiscreteKey> discrete_keys = {m1};
+  std::vector<double> leaves = {0.5, 4.3252595};
+  AlgebraicDecisionTree<Key> expected_error(discrete_keys, leaves);
+
+  EXPECT(assert_equal(expected_error, error_tree, 1e-6));
+
+  // Regression for non-tree version.
+  DiscreteValues assignment;
+  assignment[M(1)] = 0;
+  EXPECT_DOUBLES_EQUAL(0.5, mixture.error(values, assignment), 1e-8);
+  assignment[M(1)] = 1;
+  EXPECT_DOUBLES_EQUAL(4.3252595155709335, mixture.error(values, assignment), 1e-8);
+}
+
 /* ************************************************************************* */
 int main() {
   TestResult tr;
diff --git a/gtsam/hybrid/tests/testGaussianMixtureFactor.cpp b/gtsam/hybrid/tests/testGaussianMixtureFactor.cpp
index cb9068c303..ba0622ff9d 100644
--- a/gtsam/hybrid/tests/testGaussianMixtureFactor.cpp
+++ b/gtsam/hybrid/tests/testGaussianMixtureFactor.cpp
@@ -10,7 +10,7 @@
  * -------------------------------------------------------------------------- */
 
 /**
- * @file    GaussianMixtureFactor.cpp
+ * @file    testGaussianMixtureFactor.cpp
  * @brief   Unit tests for GaussianMixtureFactor
  * @author  Varun Agrawal
  * @author  Fan Jiang
@@ -135,7 +135,7 @@ TEST(GaussianMixtureFactor, Printing) {
   EXPECT(assert_print_equal(expected, mixtureFactor));
 }
 
-TEST_UNSAFE(GaussianMixtureFactor, GaussianMixture) {
+TEST(GaussianMixtureFactor, GaussianMixture) {
   KeyVector keys;
   keys.push_back(X(0));
   keys.push_back(X(1));
@@ -151,6 +151,46 @@ TEST_UNSAFE(GaussianMixtureFactor, GaussianMixture) {
   EXPECT_LONGS_EQUAL(2, gm.discreteKeys().size());
 }
 
+/* ************************************************************************* */
+// Test the error of the GaussianMixtureFactor
+TEST(GaussianMixtureFactor, Error) {
+  DiscreteKey m1(1, 2);
+
+  auto A01 = Matrix2::Identity();
+  auto A02 = Matrix2::Identity();
+
+  auto A11 = Matrix2::Identity();
+  auto A12 = Matrix2::Identity() * 2;
+
+  auto b = Vector2::Zero();
+
+  auto f0 = boost::make_shared<JacobianFactor>(X(1), A01, X(2), A02, b);
+  auto f1 = boost::make_shared<JacobianFactor>(X(1), A11, X(2), A12, b);
+  std::vector<GaussianFactor::shared_ptr> factors{f0, f1};
+
+  GaussianMixtureFactor mixtureFactor({X(1), X(2)}, {m1}, factors);
+
+  VectorValues continuousValues;
+  continuousValues.insert(X(1), Vector2(0, 0));
+  continuousValues.insert(X(2), Vector2(1, 1));
+
+  // error should return a tree of errors, with nodes for each discrete value.
+  AlgebraicDecisionTree<Key> error_tree = mixtureFactor.error(continuousValues);
+
+  std::vector<DiscreteKey> discrete_keys = {m1};
+  // Error values for regression test
+  std::vector<double> errors = {1, 4};
+  AlgebraicDecisionTree<Key> expected_error(discrete_keys, errors);
+
+  EXPECT(assert_equal(expected_error, error_tree));
+
+  // Test for single leaf given discrete assignment P(X|M,Z).
+  DiscreteValues discreteValues;
+  discreteValues[m1.first] = 1;
+  EXPECT_DOUBLES_EQUAL(
+      4.0, mixtureFactor.error(continuousValues, discreteValues), 1e-9);
+}
+
 /* ************************************************************************* */
 int main() {
   TestResult tr;
diff --git a/gtsam/hybrid/tests/testHybridBayesNet.cpp b/gtsam/hybrid/tests/testHybridBayesNet.cpp
index e1fe724695..050c01aeda 100644
--- a/gtsam/hybrid/tests/testHybridBayesNet.cpp
+++ b/gtsam/hybrid/tests/testHybridBayesNet.cpp
@@ -183,6 +183,60 @@ TEST(HybridBayesNet, OptimizeMultifrontal) {
   EXPECT(assert_equal(expectedValues, delta.continuous(), 1e-5));
 }
 
+/* ****************************************************************************/
+// Test bayes net error
+TEST(HybridBayesNet, Error) {
+  Switching s(3);
+
+  Ordering hybridOrdering = s.linearizedFactorGraph.getHybridOrdering();
+  HybridBayesNet::shared_ptr hybridBayesNet =
+      s.linearizedFactorGraph.eliminateSequential(hybridOrdering);
+
+  HybridValues delta = hybridBayesNet->optimize();
+  auto error_tree = hybridBayesNet->error(delta.continuous());
+
+  std::vector<DiscreteKey> discrete_keys = {{M(0), 2}, {M(1), 2}};
+  std::vector<double> leaves = {0.0097568009, 3.3973404e-31, 0.029126214,
+                                0.0097568009};
+  AlgebraicDecisionTree<Key> expected_error(discrete_keys, leaves);
+
+  // regression
+  EXPECT(assert_equal(expected_error, error_tree, 1e-9));
+
+  // Error on pruned bayes net
+  auto prunedBayesNet = hybridBayesNet->prune(2);
+  auto pruned_error_tree = prunedBayesNet.error(delta.continuous());
+
+  std::vector<double> pruned_leaves = {2e50, 3.3973404e-31, 2e50, 0.0097568009};
+  AlgebraicDecisionTree<Key> expected_pruned_error(discrete_keys,
+                                                   pruned_leaves);
+
+  // regression
+  EXPECT(assert_equal(expected_pruned_error, pruned_error_tree, 1e-9));
+
+  // Verify error computation and check for specific error value
+  DiscreteValues discrete_values;
+  boost::assign::insert(discrete_values)(M(0), 1)(M(1), 1);
+
+  double total_error = 0;
+  for (size_t idx = 0; idx < hybridBayesNet->size(); idx++) {
+    if (hybridBayesNet->at(idx)->isHybrid()) {
+      double error = hybridBayesNet->atMixture(idx)->error(delta.continuous(),
+                                                           discrete_values);
+      total_error += error;
+    } else if (hybridBayesNet->at(idx)->isContinuous()) {
+      double error = hybridBayesNet->atGaussian(idx)->error(delta.continuous());
+      total_error += error;
+    }
+  }
+
+  EXPECT_DOUBLES_EQUAL(
+      total_error, hybridBayesNet->error(delta.continuous(), discrete_values),
+      1e-9);
+  EXPECT_DOUBLES_EQUAL(total_error, error_tree(discrete_values), 1e-9);
+  EXPECT_DOUBLES_EQUAL(total_error, pruned_error_tree(discrete_values), 1e-9);
+}
+
 /* ****************************************************************************/
 // Test bayes net pruning
 TEST(HybridBayesNet, Prune) {
diff --git a/gtsam/hybrid/tests/testHybridGaussianFactorGraph.cpp b/gtsam/hybrid/tests/testHybridGaussianFactorGraph.cpp
index ed6b97ab04..7877461b67 100644
--- a/gtsam/hybrid/tests/testHybridGaussianFactorGraph.cpp
+++ b/gtsam/hybrid/tests/testHybridGaussianFactorGraph.cpp
@@ -562,6 +562,36 @@ TEST(HybridGaussianFactorGraph, Conditionals) {
   EXPECT(assert_equal(expected_discrete, result.discrete()));
 }
 
+/* ****************************************************************************/
+// Test hybrid gaussian factor graph error and unnormalized probabilities
+TEST(HybridGaussianFactorGraph, ErrorAndProbPrime) {
+  Switching s(3);
+
+  HybridGaussianFactorGraph graph = s.linearizedFactorGraph;
+
+  Ordering hybridOrdering = graph.getHybridOrdering();
+  HybridBayesNet::shared_ptr hybridBayesNet =
+      graph.eliminateSequential(hybridOrdering);
+
+  HybridValues delta = hybridBayesNet->optimize();
+  auto error_tree = graph.error(delta.continuous());
+
+  std::vector<DiscreteKey> discrete_keys = {{M(0), 2}, {M(1), 2}};
+  std::vector<double> leaves = {0.9998558, 0.4902432, 0.5193694, 0.0097568};
+  AlgebraicDecisionTree<Key> expected_error(discrete_keys, leaves);
+
+  // regression
+  EXPECT(assert_equal(expected_error, error_tree, 1e-7));
+
+  auto probs = graph.probPrime(delta.continuous());
+  std::vector<double> prob_leaves = {0.36793249, 0.61247742, 0.59489556,
+                                     0.99029064};
+  AlgebraicDecisionTree<Key> expected_probs(discrete_keys, prob_leaves);
+
+  // regression
+  EXPECT(assert_equal(expected_probs, probs, 1e-7));
+}
+
 /* ************************************************************************* */
 int main() {
   TestResult tr;
diff --git a/gtsam/hybrid/tests/testMixtureFactor.cpp b/gtsam/hybrid/tests/testMixtureFactor.cpp
new file mode 100644
index 0000000000..fe3212eda5
--- /dev/null
+++ b/gtsam/hybrid/tests/testMixtureFactor.cpp
@@ -0,0 +1,109 @@
+/* ----------------------------------------------------------------------------
+
+ * GTSAM Copyright 2010, Georgia Tech Research Corporation,
+ * Atlanta, Georgia 30332-0415
+ * All Rights Reserved
+ * Authors: Frank Dellaert, et al. (see THANKS for the full author list)
+
+ * See LICENSE for the license information
+
+ * -------------------------------------------------------------------------- */
+
+/**
+ * @file    testMixtureFactor.cpp
+ * @brief   Unit tests for MixtureFactor
+ * @author  Varun Agrawal
+ * @date    October 2022
+ */
+
+#include <gtsam/base/TestableAssertions.h>
+#include <gtsam/discrete/DiscreteValues.h>
+#include <gtsam/hybrid/MixtureFactor.h>
+#include <gtsam/inference/Symbol.h>
+#include <gtsam/slam/BetweenFactor.h>
+
+// Include for test suite
+#include <CppUnitLite/TestHarness.h>
+
+using namespace std;
+using namespace gtsam;
+using noiseModel::Isotropic;
+using symbol_shorthand::M;
+using symbol_shorthand::X;
+
+/* ************************************************************************* */
+// Check iterators of empty mixture.
+TEST(MixtureFactor, Constructor) {
+  MixtureFactor factor;
+  MixtureFactor::const_iterator const_it = factor.begin();
+  CHECK(const_it == factor.end());
+  MixtureFactor::iterator it = factor.begin();
+  CHECK(it == factor.end());
+}
+
+/* ************************************************************************* */
+// Test .print() output.
+TEST(MixtureFactor, Printing) {
+  DiscreteKey m1(1, 2);
+  double between0 = 0.0;
+  double between1 = 1.0;
+
+  Vector1 sigmas = Vector1(1.0);
+  auto model = noiseModel::Diagonal::Sigmas(sigmas, false);
+
+  auto f0 =
+      boost::make_shared<BetweenFactor<double>>(X(1), X(2), between0, model);
+  auto f1 =
+      boost::make_shared<BetweenFactor<double>>(X(1), X(2), between1, model);
+  std::vector<NonlinearFactor::shared_ptr> factors{f0, f1};
+
+  MixtureFactor mixtureFactor({X(1), X(2)}, {m1}, factors);
+
+  std::string expected =
+      R"(Hybrid [x1 x2; 1]
+MixtureFactor
+ Choice(1) 
+ 0 Leaf Nonlinear factor on 2 keys
+ 1 Leaf Nonlinear factor on 2 keys
+)";
+  EXPECT(assert_print_equal(expected, mixtureFactor));
+}
+
+/* ************************************************************************* */
+// Test the error of the MixtureFactor
+TEST(MixtureFactor, Error) {
+  DiscreteKey m1(1, 2);
+
+  double between0 = 0.0;
+  double between1 = 1.0;
+
+  Vector1 sigmas = Vector1(1.0);
+  auto model = noiseModel::Diagonal::Sigmas(sigmas, false);
+
+  auto f0 =
+      boost::make_shared<BetweenFactor<double>>(X(1), X(2), between0, model);
+  auto f1 =
+      boost::make_shared<BetweenFactor<double>>(X(1), X(2), between1, model);
+  std::vector<NonlinearFactor::shared_ptr> factors{f0, f1};
+
+  MixtureFactor mixtureFactor({X(1), X(2)}, {m1}, factors);
+
+  Values continuousValues;
+  continuousValues.insert<double>(X(1), 0);
+  continuousValues.insert<double>(X(2), 1);
+
+  AlgebraicDecisionTree<Key> error_tree = mixtureFactor.error(continuousValues);
+
+  std::vector<DiscreteKey> discrete_keys = {m1};
+  std::vector<double> errors = {0.5, 0};
+  AlgebraicDecisionTree<Key> expected_error(discrete_keys, errors);
+
+  EXPECT(assert_equal(expected_error, error_tree));
+}
+
+/* ************************************************************************* */
+int main() {
+  TestResult tr;
+  return TestRegistry::runAllTests(tr);
+}
+/* ************************************************************************* */