Market Proxy
Also written Market portfolio proxy · Proxy for the market portfolio
A practical stand-in for the theoretical market portfolio — usually a broad-based equity index. The choice matters: an incorrect proxy distorts beta and the security market line.
In plain language
CAPM measures every asset's risk against the market portfolio — a portfolio of all risky assets in the world. The workbook lists what that would have to include: domestic and foreign stocks and bonds, real estate, coins, stamps, art, antiques and any other marketable risky asset.
Nobody can build or measure that portfolio. It is not possible to construct in practice, so proxies are used. A market proxy is that stand-in, and broad-based equity market indices are the usual choice.
Two cautions from the workbook: there is no unanimity about which proxy to use, and an incorrect market proxy will affect both the beta risk measures and the position and slope of the SML used to evaluate portfolio performance.
How it works
Where the proxy enters the calculations:
| Calculation | How the proxy is used | Workbook reference |
|---|---|---|
| Beta | Sensitivity of a security's return to an index's return, "chosen as a proxy for market portfolio" | Chapter 3 (cost of equity) |
| CAPM required return | Rm = return on a market portfolio; "a popular broad market index is a good proxy" | Chapter 20, section 20.2.9 |
| Systematic risk | Indices serve as proxy for the market portfolio when calculating systematic risk | Chapter 12, section 12.2 |
| Security market line | Its position and slope depend on the proxy | Chapter 16, section 16.4 |
Why it matters. Beta is the covariance of a stock with the market divided by the market's variance. Change the "market" and both numbers change, so beta changes. The required return, the SML and every alpha measured against it change too — so a manager's apparent alpha can depend on the proxy chosen.
Related uses of the word proxy. The workbook also calls beta a proxy for non-diversifiable (systematic) risk, and the government security yield of similar maturity the best proxy for the risk-free rate. Those are different proxies; the market proxy specifically stands in for portfolio M.
A worked example
Illustrative figures. A PMS manager's large-cap portfolio returned 15% last year. The risk-free rate was 7%. She evaluates herself with CAPM using two possible market proxies.
| Proxy 1: broad large-cap index | Proxy 2: a narrow 30-stock index | |
|---|---|---|
| Proxy return (Rm) | 13% | 16% |
| Portfolio beta against the proxy | 1.1 | 0.9 |
| Required return = 7 + β × (Rm − 7) | 7 + 1.1 × 6 = 13.6% | 7 + 0.9 × 9 = 15.1% |
| Alpha = 15 − required | +1.4% | −0.1% |
Same portfolio, same year. Against Proxy 1 she added value; against Proxy 2 she slightly underperformed. On a ₹25 crore portfolio, the difference between the two alphas (1.5 percentage points) is about ₹37.5 lakh of apparent value added.
This is the workbook's warning in action: an incorrect proxy changes beta, the SML and the conclusion about performance.
Why NISM asks about it
Chapter 16 (Introduction to Capital Market Theory), section 16.4, explains why a proxy is needed for the market portfolio and warns about incorrect proxies. Chapter 12 (section 12.2) lists acting as a market proxy among the uses of indices — Chapter 12's sample question 1 includes measuring systematic risk among an index's uses. Chapter 20 (section 20.2.9, Alpha and Beta return) uses a broad index as the proxy in the alpha calculation.
Common exam traps
- The true market portfolio includes all risky assets worldwide — not just equities. The proxy is usually equity-only, which is why it is imperfect.
- There is no unanimity about which proxy to use.
- An incorrect proxy affects beta and the SML — both the position and the slope of the SML.
- Market proxy ≠ beta as a proxy. Beta is a proxy for systematic risk; the market proxy stands in for portfolio M.
- A proxy can change a manager's measured alpha, so performance comparisons should state which proxy was used.
Where this is taught
Free preparation for NISM Series XXI-BRelated terms
- BetaHow sharply a share moves relative to the market index — beta 1 moves with the index, above 1 amplifies it, below 1 dampens it. The standard measure of systematic risk.
- CAPMA model that prices the return an investor should demand from a share: the risk-free rate plus beta times the market risk premium.
- Systematic riskThe part of an investment's risk that comes from economy-wide forces moving every asset at once — it cannot be diversified away, and it is the only risk the market pays you to carry.
- AlphaThe return a fund earned above what its beta and the benchmark say it should have earned — the slice of performance left over once the market has been given credit for its share.
- BenchmarkThe independently published index a scheme's performance is measured against, chosen to match its investment objective, asset allocation and strategy, and disclosed in the Scheme Information Document.
- Market PortfolioThe tangency portfolio M of all risky assets worldwide in market proportions — the one risky portfolio every rational investor holds once a risk-free asset exists; indices serve as its proxy.
- Security Market LineThe graph of CAPM: expected return plotted against beta. In equilibrium every asset lies on it — a security plotting above is undervalued, one plotting below is overvalued.