The base quarter rule, and the half-point error it prevents
Nearly every published explanation of the cost-of-living adjustment says it compares this year’s third quarter to last year’s. That is right in most years and wrong in exactly the years where it matters — and SSA’s own two pages disagree about it.
The 2017 adjustment
0.3%
The naive year-over-year reading gives 0.8% — wrong by 0.5%, because the base is the third quarter of 2014, not 2015.
The rule as it is actually written
A COLA effective for December of the current year equals the percentage increase in the CPI-W from the third quarter of the last year in which a COLA became effective to the third quarter of the current year, rounded to the nearest tenth of one percent, floored at zero.
Two SSA pages, two formulations
SSA's colaseries.html says 'the third quarter of the prior year' and latestCOLA.html says 'the last year in which a COLA became effective'. These coincide except after a zero-COLA year (determinations 2011 and 2016), where the prior-year reading is wrong. The latestCOLA formulation is the operative one.
- Zero years on record
- 32016, 2011, 2010
- Years the rule bites
- 32011 and 2012 and 2017
- Largest error avoided
- 1.5%
- Deepest walk-back
- 3 years
The mechanism
When prices do not rise, the statute produces a zero rather than a negative adjustment. Benefits hold. If the base quarter then reset to the zero year, the price movement inside that year would be measured from a lower starting point and counted twice — or, depending on direction, lost entirely. So the base does not reset. It stays at the last quarter that actually produced an adjustment until a new one is produced.
The practical consequence is that a zero year defers an increase rather than cancelling it. That is a genuinely reassuring fact and it is almost never explained.
Both cases, worked from the raw index
2011 — base Q3 2008, not Q3 2009
| Method | Base quarter | Base average | Comparison average | Result |
|---|---|---|---|---|
| The rule as written | Q3 2008 | 215.495 | 214.136 | 0.0% |
| Naive year-over-year | Q3 2009 | 211.001 | 214.136 | 1.5% |
Unrounded, the rule gives -0.63% and the naive reading gives 1.49%. SSA published 0.0%. The 2011 page.
2012 — base Q3 2008, not Q3 2010
| Method | Base quarter | Base average | Comparison average | Result |
|---|---|---|---|---|
| The rule as written | Q3 2008 | 215.495 | 223.233 | 3.6% |
| Naive year-over-year | Q3 2010 | 214.136 | 223.233 | 4.2% |
Unrounded, the rule gives 3.59% and the naive reading gives 4.25%. SSA published 3.6%. The 2012 page.
2017 — base Q3 2014, not Q3 2015
| Method | Base quarter | Base average | Comparison average | Result |
|---|---|---|---|---|
| The rule as written | Q3 2014 | 234.242 | 235.057 | 0.3% |
| Naive year-over-year | Q3 2015 | 233.278 | 235.057 | 0.8% |
Unrounded, the rule gives 0.35% and the naive reading gives 0.76%. SSA published 0.3%. The 2017 page.
Why 2012 is the harder case
The 2012 walk-back crosses two consecutive zero years and lands on the third quarter of 2008 — a gap of 3 years between the base and the comparison. Code written to handle a single zero year produces a wrong answer here even after it has been corrected for the other case. This site walks backward until it finds a non-zero published adjustment, however far back that is, and throws rather than guessing if it runs off the start of the series.
Where this leaves 2026
The 2026 adjustment is not one of the affected years: its base is the ordinary prior year, so the two readings coincide. The distinction only surfaces after a zero, and it will surface again the next time one occurs. The three zero years on record.
Common questions
What exactly is the base quarter?
The third quarter of the last year in which a cost-of-living adjustment became effective. In most years that is simply the previous year, because most years produce an adjustment. After a zero year it is not.
Why do SSA’s two pages say different things?
The series page uses the shorthand "the third quarter of the prior year", which is true in every year except the one after a zero. The latest-determination page states the precise rule. The precise one is operative, and the Federal Register determination notices state it the same way.
Does a zero year mean beneficiaries lose that inflation permanently?
No — and this rule is exactly why. Because the base quarter is carried forward rather than reset, the next adjustment measures from the last quarter that produced one. Any price rise during the zero year is still inside the next comparison. The increase is deferred, not erased.
Has the walk-back ever crossed more than one zero year?
Yes. The determination made in 2011 walked back across two consecutive zero years to the third quarter of 2008. An implementation that only handles one zero year gets that case wrong even after it has been fixed for the other.
Related
- The computationfour steps
- Methodologythree regimes
- Zero years3 of them
- 2011a proof year
- 2012a proof year
- 2017a proof year
- The CPI-W seriesthe input
Sources and freshness
- SSA Office of the Chief Actuary — Latest cost-of-living adjustment — verified 10 August 2026
- SSA Office of the Chief Actuary — Cost-of-Living Adjustments — verified 10 August 2026
- U.S. Bureau of Labor Statistics — CPI-W (CWUR0000SA0), all items, U.S. city average, not seasonally adjusted — verified 10 August 2026
Page figures last verified against the sources above on 10 August 2026. BLS data retrieved 10 August 2026. BLS.gov cannot vouch for data or analyses derived from these data after retrieval. Corrections: the correction log · support@inventum.com.au