Year-on-Year Comparison

A statistical method that compares the value at a specific point in time with the value at the same point one year earlier, used to identify trends by offsetting seasonality.

Year-on-Year Comparison

Overview

Year-on-year (YoY) comparison is a statistical method that compares the value at a specific point in time with the value at the same point one year earlier. Because it offsets seasonal factors and short-term noise, making long-term trends easier to identify, it has become a standard notation for almost all economic indicators, including economic growth rates, corporate earnings, prices, and employment.

Main Content

Definition and Calculation

YoY growth is calculated as '(value in the current period − value in the same period last year) ÷ value in the same period last year × 100 (%)'. For example, if the consumer price index in March 2025 is 115 and in March 2024 is 110, the YoY increase is about 4.5%. It is customary to report the rate of change rather than the index itself, and the expressions 'year-on-year for the same month', 'year-on-year for the same quarter', and 'year-on-year for the same period' all refer to the same concept.

Scope of 'Same Period'

The 'same period' used as the comparison base depends on the indicator's frequency. Monthly indicators are compared with the same month of the previous year, quarterly indicators with the same quarter of the previous year, and semiannual or annual indicators with the cumulative same period of the previous year. Cumulative comparison (January–March cumulative versus January–March cumulative of the previous year) is common for items with large monthly fluctuations and often appears in reports on exports and imports or tax revenue.

Difference from Month-on-Month and Quarter-on-Quarter

Month-on-month (MoM) and quarter-on-quarter (QoQ) show short-term changes from the immediately preceding period, whereas YoY shows changes with the one-year seasonal pattern removed. Thus, for highly seasonal indicators (retail sales, industrial production, tourist numbers, real estate transaction volumes), YoY provides a more stable signal. However, MoM captures recent turning points faster, so in practice the two indicators are generally presented together.

Seasonality and Base Effect

The greatest strength and weakness of YoY lies in the 'base'. If the value one year earlier was unusually high or low, the growth rate is greatly distorted regardless of the current year's actual trend. This is called the base effect. For example, if there was a large one-off factor in the previous year, this year's YoY rate may move in the opposite direction from the actual economy. Therefore, when interpreting YoY figures, one should check at least two to three years of time series and the absolute level together.

Representative Use Cases

GDP growth rate: the Bank of Korea publishes both quarter-on-quarter and year-on-year figures.
Consumer price inflation: Statistics Korea's headline figure is year-on-year for the same month.
Export and import performance: the Ministry of Trade, Industry and Energy's preliminary 'first 10 days of exports' figures are also expressed year-on-year.
Corporate earnings: listed companies' revenue and operating profit growth rates are mostly compared on a YoY basis.
Employment indicators: changes in the number of employed persons are reported year-on-year for the same month.
Real estate: used to judge trends in housing price indices, transaction volumes, and unsold housing units.

Points to Note in Interpretation

First, nominal and real indicators must be distinguished. During periods of inflation, YoY growth in nominal sales can overstate actual volume growth. Second, there is the exchange rate effect. For exporters, won-denominated revenue can swing sharply in YoY terms due solely to exchange rate movements. Third, sample and composition changes. If survey targets or index methodologies are changed, comparability may be impaired. Fourth, when the business scope has changed, such as through mergers and acquisitions or business restructuring, organic growth should be checked separately.

Related Concepts

Year-on-year for the same month, quarter-on-quarter, cumulative comparison, moving average, seasonal adjustment, and compound annual growth rate (CAGR) are representative statistical concepts used together with YoY.

Recent Trends

In 2024–2025, debate over the 'base effect' became particularly prominent in interpreting YoY indicators. The base from the high-inflation and high-interest-rate phase of 2022–2023 mechanically amplified the slowdown in inflation in 2024, while conversely the base from the weak semiconductor industry in 2023 produced a sharp YoY rebound in exports and facility investment in 2024. In 2025, double-digit YoY growth in semiconductor and server revenue driven by expanded AI data center investment contrasted sharply with YoY contraction in the secondary battery and components sectors due to the EV chasm. In addition, statistical authorities and major institutions have further strengthened the practice of presenting MoM and YoY together with seasonally adjusted time series, and it has become common for users to switch directly between YoY and MoM on data platforms (KOSIS, ECOS, etc.). In practice, it is recommended to combine three-year moving averages, cumulative comparisons, and level indicators rather than relying on a single YoY figure.