Adding TransformedLogLikelihood class

CHANGELOG.md

@@ -4,8 +4,8 @@ All notable changes to this project will be documented in this file.
 
 
 ## Unreleased
-
 ### Added
+- [#1730](https://github.com/pints-team/pints/pull/1730) Added a `TransformedLogLikelihood` class which, unlike generic `TransformedLogPDF` objects, is invariant with respect to the parameters.
 - [#1724](https://github.com/pints-team/pints/pull/1724) The `LogLikelihood` class has been reintroduced, to differentiate between probabilities of parameters and probabilities of data, given fixed parameters.
 - [#1724](https://github.com/pints-team/pints/pull/1724) Added `PooledLogLikelihood` and `SumOfIndependentLogLikelihoods`.
 - [#1716](https://github.com/pints-team/pints/pull/1716) PINTS is now tested on Python 3.14.
@@ -15,10 +15,12 @@ All notable changes to this project will be documented in this file.
 - [#1724](https://github.com/pints-team/pints/pull/1724) Some methods that accepted `LogPDF`s now specifically require `LogLikelihood`s (e.g. `LogPosterior`, `NestedController`).
 - [#1713](https://github.com/pints-team/pints/pull/1713) PINTS now requires matplotlib 2.2 or newer.
 ### Deprecated
+- [#1730](https://github.com/pints-team/pints/pull/1730) The method `Transformation.convert_log_prior` is deprecated, as `convert_log_pdf` now calls the appropriate method based on the type of `LogPDF` passed in.
 - [#1724](https://github.com/pints-team/pints/pull/1724) The classes `PooledLogPDF` and `SumOfIndependentLogPDFs` are deprecated, in favour of `PooledLogLikelihood` and `SumOfIndependentLogLikelihoods` respectively.
 - [#1508](https://github.com/pints-team/pints/pull/1508) The methods `OptimisationController.max_unchanged_iterations` and `set_max_unchanged_iterations` are deprecated, in favour of `function_tolerance` and `set_function_tolerance` respectively.
 ### Removed
 ### Fixed
+- [#1730](https://github.com/pints-team/pints/pull/1730) Log-likelihoods are now transformed correctly.
 - [#1713](https://github.com/pints-team/pints/pull/1713) Fixed Numpy 2.4.1 compatibility issues.
 - [#1690](https://github.com/pints-team/pints/pull/1690) Fixed bug in optimisation controller if population size left at `None`.
 

docs/source/transformations.rst

@@ -31,6 +31,25 @@ Example::
     transform = pints.LogTransformation(n_parameters)
     mcmc = pints.MCMCController(log_posterior, n_chains, x0, transform=transform)
 
+Transformation types:
+
+ - :class:`ComposedTransformation`
+ - :class:`IdentityTransformation`
+ - :class:`LogitTransformation`
+ - :class:`LogTransformation`
+ - :class:`RectangularBoundariesTransformation`
+ - :class:`ScalingTransformation`
+ - :class:`UnitCubeTransformation`
+
+Transformed classes:
+
+ - :class:`Transformation`
+ - :class:`TransformedBoundaries`
+ - :class:`TransformedErrorMeasure`
+ - :class:`TransformedLogLikelihood`
+ - :class:`TransformedLogPDF`
+ - :class:`TransformedLogPrior`
+
 
 Transformation types
 ********************
@@ -58,6 +77,8 @@ Transformed objects
 
 .. autoclass:: TransformedErrorMeasure
 
+.. autoclass:: TransformedLogLikelihood
+
 .. autoclass:: TransformedLogPDF
 
 .. autoclass:: TransformedLogPrior

pints/__init__.py

@@ -275,6 +275,7 @@ def version(formatted=False):
     Transformation,
     TransformedBoundaries,
     TransformedErrorMeasure,
+    TransformedLogLikelihood,
     TransformedLogPDF,
     TransformedLogPrior,
     TransformedRectangularBoundaries,

pints/_optimisers/__init__.py

@@ -236,7 +236,8 @@ def xbest(self):
     def x_best(self):
         """
         Returns the best position seen during an optimisation, i.e. the point
-        for which the minimal error or maximum LogPDF was observed.
+        for which the minimal error or maximum probability density was
+        observed.
         """
         raise NotImplementedError
 
@@ -245,7 +246,7 @@ def x_guessed(self):
         Returns the optimiser's current best estimate of where the optimum is.
 
         For many optimisers, this will simply be the point for which the
-        minimal error or maximum LogPDF was observed, so that
+        minimal error or maximum probability density was observed, so that
         ``x_guessed = x_best``. However, optimisers like :class:`pints.CMAES`
         and its derivatives, maintain a separate "best guess" value that does
         not necessarily correspond to any of the points evaluated during the

pints/_transformation.py

@@ -32,25 +32,46 @@ class Transformation():
     """
     def convert_log_pdf(self, log_pdf):
         """
-        Returns a transformed log-PDF class.
+        Returns a transformed :class:`pints.LogPDF`.
+
+        If `log_pdf` is a :class:`LogPrior`, a :class:`TransformedLogPrior`
+        will be returned, which also transforms the output of the
+        :meth:`sample` method.
+
+        If `log_pdf` is a :class:`LogLikelihood`, a
+        :class:`TransformedLogLikelihood` is returned, which is assumed to be
+        invariant with respect to the transform (because it is a probability of
+        the data, not the parameters). For all other types (including
+        ``LogPrior``) a non-invariant transform is used, see
+        :class:`TransformedLogPDF` for details.
         """
+        if isinstance(log_pdf, pints.LogLikelihood):
+            return TransformedLogLikelihood(log_pdf, self)
+        if isinstance(log_pdf, pints.LogPrior):
+            return TransformedLogPrior(log_pdf, self)
         return TransformedLogPDF(log_pdf, self)
 
     def convert_log_prior(self, log_prior):
         """
-        Returns a transformed log-prior class.
+        Deprecated function: Use :meth:`convert_log_pdf` instead.
         """
+        # Deprecated on 2026-02-06
+        import warnings
+        warnings.warn(
+            'The method `convert_log_prior` is deprecated. Please use'
+            ' `convert_log_pdf` instead (which will automatically detect'
+            ' detect LogPDF subtypes).')
         return TransformedLogPrior(log_prior, self)
 
     def convert_error_measure(self, error_measure):
         """
-        Returns a transformed error measure class.
+        Returns a transformed :class:`pints.ErrorMeasure`.
         """
         return TransformedErrorMeasure(error_measure, self)
 
     def convert_boundaries(self, boundaries):
         """
-        Returns a transformed boundaries class.
+        Returns a transformed :class:`pints.Boundaries` object.
         """
         if isinstance(boundaries, pints.RectangularBoundaries):
             if self.elementwise():
@@ -1212,6 +1233,70 @@ def sample(self, n):
         return qs
 
 
+class TransformedLogLikelihood(pints.LogLikelihood):
+    r"""
+    A :class:`pints.LogLikelihood` that accepts parameters in a transformed
+    search space.
+
+    Unlike a :class:`TransformedLogPDF`, a likelihood (a measure of how well
+    the data, $\boldsymbol{x}, is explained by a model, given fixed parameters)
+    is invariant to a parameter transform (but not to a data transform),
+    and so no Jacobian term appears. Instead for some :class:`Transformation`
+    :math:`\boldsymbol{q}=\boldsymbol{f}(\boldsymbol{p})`
+
+    .. math::
+        \underset{\boldsymbol{q}}{\text{max}}(\log L(\boldsymbol{q}|\boldsymbol{x})) =
+        \underset{\boldsymbol{q}}{\text{max}}(\log L(\boldsymbol{f}^{-1}(\boldsymbol{q}|\boldsymbol{x}))).
+
+    For the first order sensitivity, the transformation is done using
+
+    .. math::
+        \frac{\partial \log L(\boldsymbol{q}|\boldsymbol{x})}{\partial q_i} &=
+        \frac{\partial \log L(\boldsymbol{f}^{-1}(\boldsymbol{q})|\boldsymbol{x})}{\partial q_i}\\
+        &= \sum_l \frac{\partial \log L(\boldsymbol{p|\boldsymbol{x}})}{\partial p_l}
+        \frac{\partial p_l}{\partial q_i}.
+
+    Extends :class:`pints.LogLikelihood`.
+
+    Parameters
+    ----------
+    log_likelihood
+        A :class:`pints.LogLikelihood`.
+    transformation
+        A :class:`pints.Transformation`.
+    """
+    def __init__(self, log_likelihood, transformation):
+        self._log_likelihood = log_likelihood
+        self._transform = transformation
+        self._n_parameters = self._log_pdf.n_parameters()
+        if self._transform.n_parameters() != self._n_parameters:
+            raise ValueError('Number of parameters for log_likelihood and '
+                             'transformation must match.')
+
+    def __call__(self, q):
+        # Compute LogLikelihood in the model space
+        return self._log_likelihood(self._transform.to_model(q))
+
+    def evaluateS1(self, q):
+        """ See :meth:`LogPDF.evaluateS1()`. """
+
+        # Get parameters in the model space
+        p = self._transform.to_model(q)
+
+        # Call evaluateS1 of LogLikelihood in the model space
+        logl, dlogl_nojac = self._log_likelihood.evaluateS1(p)
+
+        # Calculate the S1 using change of variable (see ErrorMeasure above)
+        jacobian = self._transform.jacobian(q)
+        dlogl = np.matmul(dlogl_nojac, jacobian)  # Jacobian must be 2nd term
+
+        return logl, dlogl
+
+    def n_parameters(self):
+        """ See :meth:`LogPDF.n_parameters()`. """
+        return self._n_parameters
+
+
 class UnitCubeTransformation(ScalingTransformation):
     """
     Maps a parameter space onto the unit (hyper)cube.

pints/tests/test_transformation.py

@@ -954,7 +954,8 @@ def test_transformed_log_prior(self):
         d = 2
         t = pints.LogTransformation(2)
         r = pints.UniformLogPrior([0.1, 0.1], [0.9, 0.9])
-        tr = t.convert_log_prior(r)
+        tr = t.convert_log_pdf(r)
+        self.assertIsInstance(tr, pints.TransformedLogPrior)
 
         # Test sample
         n = 1
@@ -966,6 +967,13 @@ def test_transformed_log_prior(self):
         self.assertEqual(x.shape, (n, d))
         self.assertTrue(np.all(x < 0.))
 
+        # Test deprecated alias
+        with warnings.catch_warnings(record=True) as w:
+            tr = t.convert_log_prior(r)
+        self.assertEqual(len(w), 1)
+        self.assertIn('deprecated', str(w[0].message))
+        self.assertIsInstance(tr, pints.TransformedLogPrior)
+
 
 if __name__ == '__main__':
     unittest.main()