South Korea Ex-Post Real Policy Rate (Base Rate)

The ex-post real policy rate is the Bank of Korea's base rate (7-day RP rate) minus headline CPI year-over-year inflation, namely realized inflation, representing the real return actually experienced by savers and the real cost actually borne by borrowers.

As a backward-looking measure, the ex-post real rate answers a different question than the ex-ante rate (KREARPR). Whereas the ex-ante rate measures the real rate that agents expect at the time of decision, the ex-post rate measures the real rate that agents actually experienced, and the difference between the two equals the inflation surprise:

Despite its limitations as a forward-looking indicator, the ex-post real rate has the advantage of being directly observable without model assumptions or forecasts, and it serves as a natural benchmark for evaluating the accuracy of expected inflation estimates. A negative (-) value means that savers earn a real return below inflation and that policy was accommodative in ex-post terms, while a positive (+) value means that policy was restrictive in real terms. When the ex-post and ex-ante real rates diverge persistently, it indicates systematic forecasting errors in inflation expectations, which may signal either regime change or model misspecification.

Because the base rate is an ultra-short-term rate with no term premium, the Fisher equation is applied directly without ACM decomposition.

(1) Input Definition. $i^{base}_t$ is the Bank of Korea's 7-day RP rate (the base rate), and $\pi^{realized}_t$ is the headline CPI year-over-year published by Statistics Korea.

(2) Fisher Subtraction. The ex-post real policy rate is defined as the difference between the two series:

Unlike the ex-ante real rate (KREARPR), which subtracts the model-estimated 1-year expected inflation $\pi^{short}_t$, the ex-post rate uses actually observed inflation, making it a backward-looking measure of the real policy stance. The series is stored with maturity = 0 to distinguish it from KTB-based real rates (maturity $\in \{12, 36, 60, 120\}$).

As a backward-looking measure, the ex-post real rate reveals the actual real effect of monetary policy after the fact, providing an accountability metric for evaluating central bank performance.

Significant and persistent deviations between the ex-post and ex-ante real rates indicate inflation surprises, namely situations where realized inflation differed from what was expected. During the 2022–2023 global inflation surge, many central banks experienced large positive (+) inflation surprises in which realized inflation exceeded expectations, recording deeply negative (-) ex-post real rates even though ex-ante rates had already been adjusted upward. This divergence provided a real-time signal that inflation expectations were failing to keep pace with actual price dynamics.

The ex-post real rate was the primary real rate concept used in early macroeconomic analysis before the development of model-based expected inflation estimates, and it underpinned the analysis of monetary non-neutrality (Mundell 1963; Tobin 1965). Later work showed that the ex-post rate is a poor proxy for the ex-ante rate during periods of volatile or unpredictable inflation (Mishkin 1981).

In the monetary economics literature, the ex-post real rate is central to the debate on the Fisher effect, namely whether nominal interest rates adjust one-for-one with inflation. Fama (1975) argued that the Fisher effect holds approximately in short-term interest rates, implying that ex-ante real rates are relatively stable. Subsequent work by Mishkin (1992) and others has shown that the Fisher effect holds more reliably at long horizons but can break down in the short term, particularly during monetary policy regime changes.

For fiscal policy analysis, the ex-post real rate determines the real cost of government debt. When the ex-post real rate is below the real GDP growth rate ($r^{ex\text{-}post} < g$), the debt-to-GDP ratio declines even without primary surpluses, a condition analyzed by Blanchard (2019) in the context of the '$r$ versus $g$' debate on fiscal sustainability, and Korea's fiscal sustainability assessments frequently reference the real rate–growth rate differential as well.

The ex-post real rate also captures the redistributive effects of monetary policy between savers and borrowers. The negative (-) ex-post real rates that prevailed in Korea and many other economies during 2020–2022 transferred wealth from savers to borrowers, because savers received returns below inflation while borrowers saw their real debt burden eroded. This is the effect that Keynes (1923) termed the 'euthanasia of the rentier.'

The spread between the ex-post and ex-ante real policy rates ($\text{KREPRPR} - \text{KREARPR}$) provides a direct, model-consistent measure of inflation expectations anchoring, which is valuable for bond portfolio inflation risk management.

When this spread is close to zero, inflation expectations are well-anchored, meaning that agents' forecasts closely track actual price dynamics. A persistently positive (+) spread, namely one in which the ex-post real rate exceeds the ex-ante rate, indicates that inflation has been systematically overestimated, while a negative (-) spread indicates systematic underestimation. The direction and magnitude of this spread inform the adjustment of inflation hedging positions in fixed income portfolios.

For money market investors, the ex-post real rate determines the realized real return on short-duration instruments such as bank deposits, certificates of deposit (CDs), and money market funds. In Korea's large household savings market, the ex-post real rate is the metric most directly relevant to individual savers assessing whether their deposit returns are keeping pace with the rising cost of living.

In performance attribution for fixed income portfolios, the ex-post real rate provides the realized real return benchmark for the risk-free component. Portfolio real returns can be decomposed into the real risk-free return (the ex-post real policy rate), duration return (the change in yields times duration), credit spread return, and idiosyncratic return, enabling managers to identify the sources of real value added.

For macro hedge funds and global macro strategies, the cross-country comparison of ex-post real rates identifies economies where monetary policy has been excessively loose (deeply negative real rates) or excessively tight (sharply positive real rates) relative to the business cycle, information that can inform directional rate trades, curve positioning, and FX strategies.

This is a model-derived series, the output of the ex-post Fisher decomposition with realized inflation, so the unit-root reading describes the fitted rate rather than a directly observed price, and the serial-correlation and break diagnostics run on the first difference. The series is tested over its own span from 2001-08-31 to 2026-05-31.

The ex-post real policy rate is stationary on the level, with the Dickey and Fuller (1979) and Phillips and Perron (1988) tests rejecting a unit root at p = 0.0052 and p = 0.0486 and the Kwiatkowski et al. (1992) test not rejecting stationarity. On the first difference the Ljung and Box (1978) portmanteau rejects white noise at lags 12 and 24, Q = 1776.55 and Q = 1915.61 at p = 0.000 and p = 0.000. The Bai and Perron (1998, 2003) procedure finds 4 breaks in the mean at 2008-05-31, 2012-02-29, 2016-09-30, 2022-10-31, a reading consistent with the parameter-instability inference of Andrews (1993) on the differenced object (Perron 1989).

This is a monthly model output with no posited low-integer seasonal component, so the seasonal machinery of Hylleberg et al. (1990) and Canova and Hansen (1995) carries no meaningful object and is not run (Beaulieu and Miron 1992; Ghysels and Osborn 2001).