Why Your Stocks Can Fall When the KOSPI Rises: Weights and Market Breadth
Learn why KOSPI returns can differ from your portfolio's returns through market-cap weights, index contributions, advancing stocks, and the distinction between the KOSPI and KOSPI 200.
The KOSPI aggregates stock price movements on Korea's main stock market using market-capitalization weighting, so larger companies have more influence. A rising index does not mean every stock or the average investor's account has gained. Alongside the index return, examine major stocks' contributions, sector performance, and the numbers of advancing and declining stocks.
Compare percentage changes before point changes
An index is a level relative to a base period, rather than a price in Korean won. Looking only at the number of points it moves in a day can therefore exaggerate movements when the index level is high. Calculate the percentage change from the previous close, then compare it with your holdings' returns over the same period. Despite their similar names, the KOSPI and KOSPI 200 differ in coverage and calculation rules. Comparing a product tracking the KOSPI 200 directly with the broad KOSPI can make differences in composition look like poor management, so first record the exact underlying index specified in the product documentation.
Separate large stocks' contributions from market breadth
For a simple one-day analysis, a stock's index contribution can be approximated by multiplying its previous-day weight by its daily return. Even if many stocks with small weights fall, a sharp rise in a heavily weighted stock can lift the index. The number of advancing stocks shows how broadly gains are spread across companies, while the market-cap-weighted return reflects market movements weighted by the amount of capital represented. If the two tell different stories, neither is necessarily wrong. Checking whether gains are concentrated in a particular sector can explain specifically why a portfolio focused on small caps struggles on a strong index day.
Distinguish new listings from dividends
Indexes are adjusted to maintain continuity so that changes in the number of listed shares and other nonprice factors do not mechanically cause jumps. This is why an increase in total market capitalization cannot simply be treated as a return to existing shareholders. Comparing a conventional price index with a total return index that includes dividends also produces different results. When evaluating long-term investment performance, align the starting date and dividend treatment. Read the statistical notes on the Korea Exchange index screen about whether foreign stocks are included, so you do not confuse index coverage with the scope of the market statistics shown alongside it.
Worked example
Hypothetical example: Suppose the index starts at 2,000, stock A has a 30% weight, and the remaining stocks account for 70%. If A rises 4% and the remaining group falls 1%, the index return is 0.30×4%+0.70×(-1%)=0.5%. Under the simplifying assumption of no corporate actions or weight changes, the index reaches 2,010. A decline in an account that does not hold A is entirely consistent with this result.
What to check
- Check whether the benchmark is the KOSPI or the KOSPI 200.
- Calculate both the point change and the return from the previous close.
- Distinguish major stocks' contributions from the numbers of advancers and decliners.
- Keep price-index performance separate from performance including dividends.
Sources and primary materials
This is educational material about research methods. For an actual company decision, verify the filings and primary sources for the relevant period.