MaxGhenis · Oct 11, 2026
diff --git a/changelog.d/fix-cpi-w-nsa.fixed.md b/changelog.d/fix-cpi-w-nsa.fixed.md
@@ -0,0 +1 @@
+Use the not seasonally adjusted CPI-W (BLS CWUR0000SA0, the series SSA uses for cost-of-living adjustments) for January 2024 through June 2026, where seasonally adjusted values had replaced it, and re-anchor the CPI-W projections on the not seasonally adjusted June 2026 ratio to CPI-U.
diff --git a/policyengine_us/parameters/gov/bls/cpi/cpi_w.yaml b/policyengine_us/parameters/gov/bls/cpi/cpi_w.yaml
@@ -1,4 +1,4 @@
-description: The Bureau of Labor Statistics estimates this Consumer Price Index for Urban Wage Earners and Clerical Workers.
+description: The Bureau of Labor Statistics estimates this Consumer Price Index for Urban Wage Earners and Clerical Workers (U.S. city average, all items, not seasonally adjusted).
 values:
   1913-01-01: 9.9
   1913-02-01: 9.8
@@ -1332,87 +1332,93 @@ values:
   2023-10-01: 302.071
   2023-11-01: 301.224
   2023-12-01: 300.728
-  2024-01-01: 303.733
-  2024-02-01: 305.121
-  2024-03-01: 306.349
-  2024-04-01: 307.211
-  2024-05-01: 307.233
-  2024-06-01: 307.010
-  2024-07-01: 307.410
-  2024-08-01: 307.843
-  2024-09-01: 308.539
-  2024-10-01: 309.161
-  2024-11-01: 309.945
-  2024-12-01: 311.162
-  2025-01-01: 312.776
-  2025-02-01: 313.323
-  2025-03-01: 313.175
-  2025-04-01: 313.729
-  2025-05-01: 313.961
-  2025-06-01: 314.858
-  2025-07-01: 315.279
-  2025-08-01: 316.530
-  2025-09-01: 317.477
+  2024-01-01: 302.201
+  2024-02-01: 304.284
+  2024-03-01: 306.502
+  2024-04-01: 307.811
+  2024-05-01: 308.163
+  2024-06-01: 308.054
+  2024-07-01: 308.501
+  2024-08-01: 308.640
+  2024-09-01: 309.046
+  2024-10-01: 309.358
+  2024-11-01: 308.998
+  2024-12-01: 309.067
+  2025-01-01: 311.172
+  2025-02-01: 312.460
+  2025-03-01: 313.250
+  2025-04-01: 314.243
+  2025-05-01: 314.839
+  2025-06-01: 315.945
+  2025-07-01: 316.349
+  2025-08-01: 317.306
+  2025-09-01: 318.139
   # October 2025 CPI was not published (BLS canceled the release); the
   # September value carries forward at October instants.
-  2025-11-01: 318.419
-  2025-12-01: 319.139
-  2026-01-01: 319.434
-  2026-02-01: 320.266
-  2026-03-01: 323.698
-  2026-04-01: 325.892
-  2026-05-01: 327.694
-  2026-06-01: 325.641
+  2025-11-01: 317.414
+  2025-12-01: 317.014
+  2026-01-01: 317.942
+  2026-02-01: 319.422
+  2026-03-01: 323.500
+  2026-04-01: 326.541
+  2026-05-01: 328.829
+  2026-06-01: 327.075
   # CBO projections from CBO Feb 2026 forecast for future periods.
-  # CPI-W derived from CBO CPI-U quarterly projections using ratio
-  # of last actual CPI-W (316.530, Aug 2025) to CPI-U (323.364, Aug 2025).
-  2026-08-01: 325.939 # 2026Q3: 332.976 * 316.530 / 323.364
-  2026-11-01: 328.049 # 2026Q4: 335.132 * 316.530 / 323.364
-  2027-02-01: 330.086 # 2027Q1: 337.213 * 316.530 / 323.364
-  2027-05-01: 332.080 # 2027Q2: 339.250 * 316.530 / 323.364
-  2027-08-01: 334.068 # 2027Q3: 341.281 * 316.530 / 323.364
-  2027-11-01: 336.043 # 2027Q4: 343.298 * 316.530 / 323.364
-  2028-02-01: 338.007 # 2028Q1: 345.305 * 316.530 / 323.364
-  2028-05-01: 339.963 # 2028Q2: 347.303 * 316.530 / 323.364
-  2028-08-01: 341.909 # 2028Q3: 349.291 * 316.530 / 323.364
-  2028-11-01: 343.856 # 2028Q4: 351.280 * 316.530 / 323.364
-  2029-02-01: 345.803 # 2029Q1: 353.269 * 316.530 / 323.364
-  2029-05-01: 347.754 # 2029Q2: 355.262 * 316.530 / 323.364
-  2029-08-01: 349.710 # 2029Q3: 357.260 * 316.530 / 323.364
-  2029-11-01: 351.672 # 2029Q4: 359.265 * 316.530 / 323.364
-  2030-02-01: 353.644 # 2030Q1: 361.279 * 316.530 / 323.364
-  2030-05-01: 355.626 # 2030Q2: 363.304 * 316.530 / 323.364
-  2030-08-01: 357.619 # 2030Q3: 365.340 * 316.530 / 323.364
-  2030-11-01: 359.622 # 2030Q4: 367.386 * 316.530 / 323.364
-  2031-02-01: 361.636 # 2031Q1: 369.444 * 316.530 / 323.364
-  2031-05-01: 363.660 # 2031Q2: 371.512 * 316.530 / 323.364
-  2031-08-01: 365.695 # 2031Q3: 373.591 * 316.530 / 323.364
-  2031-11-01: 367.742 # 2031Q4: 375.682 * 316.530 / 323.364
-  2032-02-01: 369.800 # 2032Q1: 377.784 * 316.530 / 323.364
-  2032-05-01: 371.869 # 2032Q2: 379.898 * 316.530 / 323.364
-  2032-08-01: 373.950 # 2032Q3: 382.024 * 316.530 / 323.364
-  2032-11-01: 376.042 # 2032Q4: 384.161 * 316.530 / 323.364
-  2033-02-01: 378.146 # 2033Q1: 386.310 * 316.530 / 323.364
-  2033-05-01: 380.261 # 2033Q2: 388.471 * 316.530 / 323.364
-  2033-08-01: 382.388 # 2033Q3: 390.644 * 316.530 / 323.364
-  2033-11-01: 384.528 # 2033Q4: 392.830 * 316.530 / 323.364
-  2034-02-01: 386.678 # 2034Q1: 395.027 * 316.530 / 323.364
-  2034-05-01: 388.842 # 2034Q2: 397.237 * 316.530 / 323.364
-  2034-08-01: 391.018 # 2034Q3: 399.460 * 316.530 / 323.364
-  2034-11-01: 393.206 # 2034Q4: 401.695 * 316.530 / 323.364
-  2035-02-01: 395.406 # 2035Q1: 403.943 * 316.530 / 323.364
+  # CPI-W derived from CBO CPI-U quarterly projections using the ratio of
+  # the last actual CPI-W (327.075, June 2026) to CPI-U (333.952, June 2026),
+  # both not seasonally adjusted (BLS CWUR0000SA0 and CUUR0000SA0).
+  2026-08-01: 326.119 # 2026Q3: 332.976 * 327.075 / 333.952
+  2026-11-01: 328.231 # 2026Q4: 335.132 * 327.075 / 333.952
+  2027-02-01: 330.269 # 2027Q1: 337.213 * 327.075 / 333.952
+  2027-05-01: 332.264 # 2027Q2: 339.250 * 327.075 / 333.952
+  2027-08-01: 334.253 # 2027Q3: 341.281 * 327.075 / 333.952
+  2027-11-01: 336.229 # 2027Q4: 343.298 * 327.075 / 333.952
+  2028-02-01: 338.194 # 2028Q1: 345.305 * 327.075 / 333.952
+  2028-05-01: 340.151 # 2028Q2: 347.303 * 327.075 / 333.952
+  2028-08-01: 342.098 # 2028Q3: 349.291 * 327.075 / 333.952
+  2028-11-01: 344.046 # 2028Q4: 351.280 * 327.075 / 333.952
+  2029-02-01: 345.994 # 2029Q1: 353.269 * 327.075 / 333.952
+  2029-05-01: 347.946 # 2029Q2: 355.262 * 327.075 / 333.952
+  2029-08-01: 349.903 # 2029Q3: 357.260 * 327.075 / 333.952
+  2029-11-01: 351.867 # 2029Q4: 359.265 * 327.075 / 333.952
+  2030-02-01: 353.839 # 2030Q1: 361.279 * 327.075 / 333.952
+  2030-05-01: 355.823 # 2030Q2: 363.304 * 327.075 / 333.952
+  2030-08-01: 357.817 # 2030Q3: 365.340 * 327.075 / 333.952
+  2030-11-01: 359.821 # 2030Q4: 367.386 * 327.075 / 333.952
+  2031-02-01: 361.836 # 2031Q1: 369.444 * 327.075 / 333.952
+  2031-05-01: 363.862 # 2031Q2: 371.512 * 327.075 / 333.952
+  2031-08-01: 365.898 # 2031Q3: 373.591 * 327.075 / 333.952
+  2031-11-01: 367.946 # 2031Q4: 375.682 * 327.075 / 333.952
+  2032-02-01: 370.004 # 2032Q1: 377.784 * 327.075 / 333.952
+  2032-05-01: 372.075 # 2032Q2: 379.898 * 327.075 / 333.952
+  2032-08-01: 374.157 # 2032Q3: 382.024 * 327.075 / 333.952
+  2032-11-01: 376.250 # 2032Q4: 384.161 * 327.075 / 333.952
+  2033-02-01: 378.355 # 2033Q1: 386.310 * 327.075 / 333.952
+  2033-05-01: 380.471 # 2033Q2: 388.471 * 327.075 / 333.952
+  2033-08-01: 382.600 # 2033Q3: 390.644 * 327.075 / 333.952
+  2033-11-01: 384.741 # 2033Q4: 392.830 * 327.075 / 333.952
+  2034-02-01: 386.892 # 2034Q1: 395.027 * 327.075 / 333.952
+  2034-05-01: 389.057 # 2034Q2: 397.237 * 327.075 / 333.952
+  2034-08-01: 391.234 # 2034Q3: 399.460 * 327.075 / 333.952
+  2034-11-01: 393.423 # 2034Q4: 401.695 * 327.075 / 333.952
+  2035-02-01: 395.625 # 2035Q1: 403.943 * 327.075 / 333.952
   # Values for 2036-2100 are set programmatically in uprating_extensions.py
 metadata:
   unit: currency-USD
   label: CPI-W
   economy: false
   reference:
-    - title: BLS | CPI-W, U.S. city average, all items, seasonally adjusted (CWSR0000SA0), monthly values through June 2026
-      href: https://data.bls.gov/timeseries/CWSR0000SA0
+    - title: BLS | CPI-W, U.S. city average, all items, not seasonally adjusted (CWUR0000SA0), monthly values
+      href: https://data.bls.gov/timeseries/CWUR0000SA0
+    - title: SSA | CPI-W monthly values, the series behind the Social Security cost-of-living adjustment
+      href: https://www.ssa.gov/oact/STATS/cpiw.html
+    - title: BLS | CPI-U, U.S. city average, all items, not seasonally adjusted (CUUR0000SA0), June 2026 value for the projection ratio
+      href: https://data.bls.gov/timeseries/CUUR0000SA0
     - title: CBO | Economic and Budget Projections | January 2025
       # Quarterly: Row 51
       href: https://www.cbo.gov/system/files/2025-01/51135-2025-01-Economic-Projections.xlsx
     - title: CBO | Economic Projections | February 2026
       # CPI-W derived from CPI-U quarterly projections (Sheet: 1. Quarterly, Row 53)
-      # using ratio of last actual CPI-W to CPI-U (316.530 / 323.364)
+      # using the ratio of the last actual CPI-W to CPI-U, both not seasonally
+      # adjusted (327.075 / 333.952, June 2026)
       href: https://www.cbo.gov/system/files/2026-02/51135-2026-02-Economic-Projections.xlsx
diff --git a/policyengine_us/tests/policy/baseline/gov/bls/test_cpi_monthly_actuals.py b/policyengine_us/tests/policy/baseline/gov/bls/test_cpi_monthly_actuals.py
@@ -2,6 +2,10 @@
 including the October 2025 gap (never published) and the replaced
 projection points (#9091)."""
 
+from decimal import ROUND_HALF_UP, Decimal
+
+import yaml
+
 from policyengine_us.system import system
 
 
@@ -11,12 +15,14 @@ def _cpi(name):
 
 def test_cpi_w_monthly_actuals():
     cpi_w = _cpi("cpi_w")
-    # June 2026 observed value (CWSR0000SA0).
-    assert cpi_w("2026-06-01") == 325.641
+    # June 2026 observed value (CWUR0000SA0, not seasonally adjusted).
+    assert cpi_w("2026-06-01") == 327.075
     # October 2025 was never published: September carries forward.
-    assert cpi_w("2025-10-01") == 317.477
+    assert cpi_w("2025-10-01") == 318.139
     # The observed February replaces the derived 2026Q1 projection point.
-    assert cpi_w("2026-02-01") == 320.266
+    assert cpi_w("2026-02-01") == 319.422
+    # The seasonally adjusted series (CWSR0000SA0) reads 303.623 here.
+    assert cpi_w("2024-01-01") == 302.201
 
 
 def test_cpi_u_monthly_actuals():
@@ -32,3 +38,57 @@ def test_c_cpi_u_monthly_actuals():
     assert c_cpi_u("2025-10-01") == 180.196
     # The observed February replaces the annual projection point (183.1).
     assert c_cpi_u("2026-02-01") == 181.080
+
+
+def _listed_values(parameter):
+    """The dated values a parameter's YAML file lists, before the system
+    backdates the first one and extends the series."""
+    with open(parameter.file_path) as file:
+        values = yaml.safe_load(file)["values"]
+    return {str(instant): value for instant, value in values.items()}
+
+
+def _rounded_percent_increase(new, old):
+    """Percentage increase rounded to the nearest tenth, per 415(i)(1)(D)."""
+    percent = Decimal(repr(new)) / Decimal(repr(old)) * 100 - 100
+    return percent.quantize(Decimal("0.1"), ROUND_HALF_UP)
+
+
+def test_cpi_w_third_quarter_averages_reproduce_ssa_uprating():
+    """CPI-W is one series: the not seasonally adjusted index SSA uses.
+
+    gov.ssa.uprating holds the CPI-W third-quarter average (42 U.S.C.
+    415(i)(1)(A)(i) and (G)) of the latest cost-of-living computation
+    quarter. A quarter whose CPI increase, rounded to the nearest tenth of a
+    percent (415(i)(1)(D)), is not above zero is not one (415(i)(1)(B)), so
+    the level carries forward. Each year whose July-September the CPI-W file
+    lists as monthly values must reproduce SSA's value to its three
+    published decimals. The seasonally adjusted values the file held for
+    2024 failed this: they put the 2024Q3 average at 307.931, not 308.729.
+    """
+    cpi_w = _listed_values(_cpi("cpi_w"))
+    ssa_uprating = _listed_values(system.parameters.gov.ssa.uprating)
+    checked = []
+    prior_level = None
+    for instant in sorted(ssa_uprating):
+        year = int(instant[:4])
+        months = [f"{year - 1}-{month:02d}-01" for month in (7, 8, 9)]
+        if not all(month in cpi_w for month in months):
+            # CBO-based projections from here on.
+            break
+        average = sum(cpi_w[month] for month in months) / 3
+        level = ssa_uprating[instant]
+        if prior_level is None or _rounded_percent_increase(average, prior_level) > 0:
+            assert abs(level - average) < 0.0005 + 1e-9, (
+                f"gov.ssa.uprating {year} is {level}, but the {year - 1} "
+                f"third-quarter CPI-W average is {average:.4f}."
+            )
+        else:
+            assert level == prior_level, (
+                f"No cost-of-living increase for {year}, so gov.ssa.uprating "
+                f"should stay at {prior_level}, not {level}."
+            )
+        prior_level = level
+        checked.append(year)
+    # The 2022-2026 actuals, through the 2025Q3 average SSA used for 2026.
+    assert set(range(2022, 2027)) <= set(checked), checked
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Use the not seasonally adjusted CPI-W (BLS CWUR0000SA0, the series SSA uses for cost-of-living adjustments) for January 2024 through June 2026, where seasonally adjusted values had replaced it, and re-anchor the CPI-W projections on the not seasonally adjusted June 2026 ratio to CPI-U.