Add jitter_scale parameter for initial point generation

pymc/initial_point.py

@@ -66,6 +66,7 @@ def make_initial_point_fns_per_chain(
     model,
     overrides: StartDict | Sequence[StartDict | None] | None,
     jitter_rvs: set[TensorVariable] | None = None,
+    jitter_scale: float = 1.0,
     chains: int,
 ) -> list[Callable]:
     """Create an initial point function for each chain, as defined by initvals.
@@ -96,6 +97,7 @@ def make_initial_point_fns_per_chain(
                 model=model,
                 overrides=overrides,
                 jitter_rvs=jitter_rvs,
+                jitter_scale=jitter_scale,
                 return_transformed=True,
             )
         ] * chains
@@ -104,6 +106,7 @@ def make_initial_point_fns_per_chain(
             make_initial_point_fn(
                 model=model,
                 jitter_rvs=jitter_rvs,
+                jitter_scale=jitter_scale,
                 overrides=chain_overrides,
                 return_transformed=True,
             )
@@ -122,6 +125,7 @@ def make_initial_point_fn(
     model,
     overrides: StartDict | None = None,
     jitter_rvs: set[TensorVariable] | None = None,
+    jitter_scale: float = 1.0,
     default_strategy: str = "support_point",
     return_transformed: bool = True,
 ) -> Callable:
@@ -150,6 +154,7 @@ def make_initial_point_fn(
         rvs_to_transforms=model.rvs_to_transforms,
         initval_strategies=initval_strats,
         jitter_rvs=jitter_rvs,
+        jitter_scale=jitter_scale,
         default_strategy=default_strategy,
         return_transformed=return_transformed,
     )
@@ -188,6 +193,7 @@ def make_initial_point_expression(
     rvs_to_transforms: dict[TensorVariable, Transform],
     initval_strategies: dict[TensorVariable, np.ndarray | Variable | str | None],
     jitter_rvs: set[TensorVariable] | None = None,
+    jitter_scale: float = 1.0,
     default_strategy: str = "support_point",
     return_transformed: bool = False,
 ) -> list[TensorVariable]:
@@ -265,7 +271,7 @@ def make_initial_point_expression(
             value = transform.forward(value, *variable.owner.inputs)
 
         if variable in jitter_rvs:
-            jitter = pt.random.uniform(-1, 1, size=value.shape)
+            jitter = pt.random.uniform(-jitter_scale, jitter_scale, size=value.shape)
             jitter.name = f"{variable.name}_jitter"
             value = value + jitter
 

pymc/sampling/mcmc.py

@@ -1423,10 +1423,11 @@ def _init_jitter(
     initvals: StartDict | Sequence[StartDict | None] | None,
     seeds: Sequence[int] | np.ndarray,
     jitter: bool,
+    jitter_scale: float,
     jitter_max_retries: int,
     logp_dlogp_func=None,
 ) -> list[PointType]:
-    """Apply a uniform jitter in [-1, 1] to the test value as starting point in each chain.
+    """Apply a uniform jitter in [-jitter_scale, jitter_scale] to the test value as starting point in each chain.
 
     ``model.check_start_vals`` is used to test whether the jittered starting
     values produce a finite log probability. Invalid values are resampled
@@ -1449,6 +1450,7 @@ def _init_jitter(
         model=model,
         overrides=initvals,
         jitter_rvs=set(model.free_RVs) if jitter else set(),
+        jitter_scale=jitter_scale if jitter else 1.0,
         chains=len(seeds),
     )
 

tests/test_initial_point.py

@@ -152,6 +152,34 @@ def test_adds_jitter(self):
         assert fn(0) == fn(0)
         assert fn(0) != fn(1)
 
+    def test_jitter_scale(self):
+        with pm.Model() as pmodel:
+            A = pm.HalfFlat("A", initval="support_point")
+
+        jitter_scale_tests = np.array([1.0, 2.0, 5.0])
+        fns = []
+        for jitter_scale in jitter_scale_tests:
+            fns.append(
+                make_initial_point_fn(
+                    model=pmodel,
+                    jitter_rvs=set(pmodel.free_RVs),
+                    jitter_scale=jitter_scale,
+                    return_transformed=True,
+                )
+            )
+
+        n_draws = 1000
+        jitter_samples = np.empty((n_draws, len(fns)))
+        for j, fn in enumerate(fns):
+            # start and end to ensure random samples, otherwise jitter_samples across different jitter_scale will be an exact scale of each other
+            start = j * n_draws
+            end = start + n_draws
+            jitter_samples[:, j] = np.asarray([fn(i)["A_log__"] for i in range(start, end)])
+
+        init_standardised = np.mean((jitter_samples / jitter_scale_tests), axis=0)
+
+        assert np.all((-0.05 < init_standardised) & (init_standardised < 0.05))
+
     def test_respects_overrides(self):
         with pm.Model() as pmodel:
             A = pm.Flat("A", initval="support_point")