FertilityCrudeBirthRate scaling and age min check (#577)

Category: bugfix
JIRA issue: https://jira.ihme.washington.edu/browse/MIC-5915

Changes and notes
Use initialization age max instead of untracking age when determining GBD population size of the population demographic restricted to the sim.
Remove check that untracking age equals exit age.

Testing
Ran pytest.

Author: hussain-jafari

CHANGELOG.rst

@@ -1,3 +1,7 @@
+**4.3.18 - 01/06/26**
+
+  - Bugfix: Fix population scaling in FertilityCrudeBirthRate
+
 **4.3.17 - 12/18/25**
 
   - Bugfix: Fix failing doc builds

src/vivarium_public_health/population/add_new_birth_cohorts.py

@@ -111,6 +111,12 @@ class FertilityCrudeBirthRate(Component):
     #####################
 
     def setup(self, builder: Builder) -> None:
+        config = builder.configuration.population
+        if config.initialization_age_min != 0:
+            raise ValueError(
+                "Configuration key 'initialization_age_min' must be 0 if using FertilityCrudeBirthRate. "
+                f"Provided value: {config.initialization_age_min}"
+            )
         self.birth_rate = get_live_births_per_year(builder)
 
         self.clock = builder.time.clock()

src/vivarium_public_health/population/data_transformations.py

@@ -566,31 +566,25 @@ def get_live_births_per_year(builder):
 
 
 def rescale_final_age_bin(builder, population_data):
-    untracking_age = builder.configuration.population.to_dict().get("untracking_age", None)
-    if untracking_age:
+    initialization_age_max = builder.configuration.population.to_dict().get(
+        "initialization_age_max", None
+    )
+
+    if initialization_age_max:
         population_data = population_data.loc[
-            population_data["age_start"] < untracking_age
+            population_data["age_start"] < initialization_age_max
         ].copy()
-        cut_bin_idx = untracking_age <= population_data["age_end"]
+        cut_bin_idx = initialization_age_max <= population_data["age_end"]
         cut_age_start = population_data.loc[cut_bin_idx, "age_start"]
         cut_age_end = population_data.loc[cut_bin_idx, "age_end"]
-        population_data.loc[cut_bin_idx, "value"] *= (untracking_age - cut_age_start) / (
-            cut_age_end - cut_age_start
-        )
-        population_data.loc[cut_bin_idx, "age_end"] = untracking_age
+        population_data.loc[cut_bin_idx, "value"] *= (
+            initialization_age_max - cut_age_start
+        ) / (cut_age_end - cut_age_start)
+        population_data.loc[cut_bin_idx, "age_end"] = initialization_age_max
     return population_data
 
 
 def validate_crude_birth_rate_data(builder, data_year_max):
-    population_config = builder.configuration.population.to_dict()
-    untracking_age = population_config.get("untracking_age", None)
-    age_end = population_config.get("age_end", None)
-    if untracking_age and age_end and age_end != untracking_age:
-        raise ValueError(
-            "If you specify an exit age, the initial population age end must be the same "
-            "for the crude birth rate calculation to work."
-        )
-
     exceeds_data = builder.configuration.time.end.year > data_year_max
     if exceeds_data and not builder.configuration.interpolation.extrapolate:
         raise ValueError("Trying to extrapolate beyond the end of available birth data.")

tests/population/test_add_new_birth_cohort.py

@@ -52,7 +52,7 @@ def test_FertilityDeterministic(config):
 
     config.update(
         {"fertility": {"number_of_new_simulants_each_year": annual_new_simulants}},
-        **metadata(__file__)
+        **metadata(__file__),
     )
 
     components = [TestPopulation(), FertilityDeterministic()]
@@ -131,7 +131,7 @@ def test_FertilityCrudeBirthRate_extrapolate(base_config, base_plugins):
         }
     )
     pop_size = base_config.population.population_size
-    true_pop_size = 25000  # What's available in the mock artifact
+    true_pop_size = 20_000  # What's available in the mock artifact
     live_births_by_sex = 500
     components = [TestPopulation(), FertilityCrudeBirthRate()]
 
@@ -157,6 +157,31 @@ def test_FertilityCrudeBirthRate_extrapolate(base_config, base_plugins):
     np.testing.assert_allclose(birth_rate, given_birth_rate, atol=0.01)
 
 
+@pytest.mark.parametrize("initialization_age_min", [0.05, 1, 10])
+def test_FertilityCrudeBirthRate_with_non_zero_initialization_age_min_error(
+    base_config, base_plugins, initialization_age_min
+):
+    components = [TestPopulation(), FertilityCrudeBirthRate()]
+    base_config.update(
+        {
+            "population": {
+                "initialization_age_min": initialization_age_min,
+            },
+        },
+        layer="override",
+    )
+
+    with pytest.raises(
+        ValueError,
+        match=f"'initialization_age_min' must be 0 if using FertilityCrudeBirthRate. Provided value: {initialization_age_min}",
+    ):
+        simulation = InteractiveContext(
+            components=components,
+            configuration=base_config,
+            plugin_configuration=base_plugins,
+        )
+
+
 def test_fertility_module(base_config, base_plugins):
     start_population_size = 1000
     num_days = 1000