Update IFA/MMS shift docs

docs/source/models/intervention_models/mncnh_pregnancy/oral_iron_antenatal/oral_iron_antenatal.rst

@@ -377,8 +377,6 @@ Assumptions and limitations
 
 - In the case of IFA, we assume that the entire distribution experiences the same constant shift in gestational age. It is more likely that the lower end of the distribution experiences a greater shift and that the upper end experiences little to no shift (as supported from the MMS evidence). This limitation will result in an underestimation of the impact of the lower end of the distribution (which has a high magnitude of risk, but a low overall exposure) and an overestimation of the impact on the upper end of the distribution (which has lower risk magnitude, but higher overall exposure). However, we have limited data on how to better apply such a shift.
 
-- Our calculation of the IFA and MMS gestational age shifts does not take into account the correlation between ANC attendance (and therefore the population eligible to receive these interventions) and gestational age exposure that is induced through the correlated propensities used in the :ref:`facility choice model <2024_facility_model_vivarium_mncnh_portfolio>` in the MNCNH portfolio simulation. Given that there is a positive modeled correlation between these variables, there will be slightly less preterm birth among the population eligible for these interventions than among the population overall in our simulation. Therefore, our estimates of the gestational age shifts are likely less than they would be if they were calculated with consideration to this underyling correlation. Additionally, the modeled correlation in our simulation not considered in the calculation of the shifts may cause us to not exactly replicate the observed intervention RR on preterm birth that our modeling strategy aims to replicate.
-
 - We assume no effect modification by when in pregnancy oral iron is received. In reality, the effect on gestational age is likely greater for those who have taken oral iron for longer.
 
 - Our baseline calibration preserves the population mean value of gestational age at birth, but only approximates the overall exposure distribution.
@@ -434,10 +432,20 @@ The same instructions written in pseduocode in the Birthweight_ section can be f
     - Subpopulation
     - Shift
     - Note
+  * - None
+    - Overall baseline population
+    - Not covered by IFA at baseline
+    - -WEIGHTED_AVG_SHIFT * baseline_ifa_overall
+    - 
+  * - IFA
+    - Overall baseline population
+    - Non ANC attendees covered by IFA at baseline
+    - -WEIGHTED_AVG_SHIFT * baseline_ifa_overall + SHIFT_FALSE
+    - Hypothetical shift for calibration - only ANC attendees receive IFA
   * - IFA
-    - No supplementation
-    - Overall
-    - IFA_SHIFT
+    - Overall baseline population
+    - ANC attendees covered by IFA at baseline
+    - -WEIGHTED_AVG_SHIFT * baseline_ifa_overall + SHIFT_TRUE
     - 
   * - MMS
     - IFA
@@ -457,13 +465,23 @@ Where,
 
   * - Parameter
     - Value
-  * - IFA_SHIFT
-    - `Location-specific IFA_SHIFT values can be found here <https://github.com/ihmeuw/vivarium_gates_mncnh/blob/2bb721ab7b99ca60e284a0a3a948e6504d639a6d/src/vivarium_gates_mncnh/data/ifa_mms_gestation_shifts/ifa_ga_shifts.csv>`_
+  * - WEIGHTED_AVG_SHIFT
+    - SHIFT_TRUE * ANC1 + SHIFT_FALSE * (1 - ANC1)
+  * - SHIFT_TRUE
+    - IFA shift for ANC attendees.
+      `Location-specific IFA_SHIFT values can be found here <https://github.com/ihmeuw/vivarium_gates_mncnh/blob/2bb721ab7b99ca60e284a0a3a948e6504d639a6d/src/vivarium_gates_mncnh/data/ifa_mms_gestation_shifts/ifa_ga_shifts.csv>`_
+  * - SHIFT_FALSE
+    - IFA shift for non-ANC attendees.
+      `Location-specific IFA_SHIFT values can be found here <https://github.com/ihmeuw/vivarium_gates_mncnh/blob/2bb721ab7b99ca60e284a0a3a948e6504d639a6d/src/vivarium_gates_mncnh/data/ifa_mms_gestation_shifts/ifa_ga_shifts.csv>`_
   * - MMS_SHIFT_1
     - `Location-specific .csv files of MMS_SHIFT_1 values can be found here <https://github.com/ihmeuw/vivarium_gates_mncnh/blob/2bb721ab7b99ca60e284a0a3a948e6504d639a6d/src/vivarium_gates_mncnh/data/ifa_mms_gestation_shifts/updated_mms_shifts.csv>`_
   * - MMS_SHIFT_2
     - `Location-specific .csv files of MMS_SHIFT_2 values can be found here <https://github.com/ihmeuw/vivarium_gates_mncnh/blob/2bb721ab7b99ca60e284a0a3a948e6504d639a6d/src/vivarium_gates_mncnh/data/ifa_mms_gestation_shifts/updated_mms_shifts.csv>`_
 
+
+The IFA shifts above are relative to the overall baseline population-level GBD exposure values. Individually for each simulant we apply 
+the shifts to the exposure value sampled from GBD for that simulant. 
+
 .. todo:: 
 
   The values in these CSVs rely on the GBD 2021 LBWSG exposure distribution and ANC1 covariates and therefore will need to be updated when the GBD 2023 estimates are available.