What are the main equity index construction methods and how do they differ?

Question

AcadiFi · Accepted Answer

Equity indices track the performance of a basket of stocks, but the weighting method dramatically affects returns and behavior.

**Three Main Methods:**

```mermaid
graph TD
    A["Index Construction"] --> B["Price-Weighted"]
    A --> C["Market-Cap Weighted"]
    A --> D["Equal-Weighted"]
    B --> B1["Weight = Stock Price
e.g., DJIA"]
    C --> C1["Weight = Mkt Cap
e.g., S&P 500"]
    D --> D1["Weight = 1/N
Periodic Rebalancing"]
    style A fill:#c9a84c,color:#000
```

**1. Price-Weighted Index (e.g., Dow Jones Industrial Average)**
- Each stock's weight is proportional to its **share price**
- A $200 stock has 4x the influence of a $50 stock regardless of company size
- Calculated by summing all prices and dividing by a **divisor** (adjusted for splits and changes)
- **Stock split impact:** A 2:1 split halves a stock's price, reducing its weight. The divisor is adjusted so the index level stays the same.
- **Bias:** Toward high-priced stocks, not necessarily the largest companies

**2. Market-Cap Weighted (e.g., S&P 500, MSCI World)**
- Each stock's weight = its market capitalization / total market cap of all constituents
- A $500 billion company has 10x the weight of a $50 billion company
- **No rebalancing needed** for price changes — weights adjust naturally as prices move
- **Stock split impact:** None. Market cap = price x shares, and a split leaves market cap unchanged
- **Bias:** Toward mega-cap stocks. The top 10 names in the S&P 500 can represent 30%+ of the index
- **Variant:** Free-float adjusted — only publicly available shares count, excluding insider and government holdings

**3. Equal-Weighted**
- Each stock gets the same weight: 1/N where N = number of stocks
- A $10 billion company and a $1 trillion company both get equal influence
- **Requires periodic rebalancing** (quarterly or monthly) because price movements shift weights
- **Stock split impact:** None directly, but rebalancing resets any drift
- **Bias:** Toward smaller-cap stocks relative to cap-weighted; higher turnover costs

**Numerical Example — 3-Stock Index:**

| Stock | Price | Shares (M) | Mkt Cap ($B) | Price Wt | Cap Wt | Equal Wt |
|---|---|---|---|---|---|---|
| Vantage Energy | $150 | 200 | $30 | 50.0% | 42.9% | 33.3% |
| Sterling Motors | $100 | 400 | $40 | 33.3% | 57.1% | 33.3% |
| Apex Retail | $50 | 100 | $5 | 16.7% | 7.1% (Note: actually recalc -> total $75B) | 33.3% |

Notice how Vantage Energy dominates the price-weighted index despite being smaller than Sterling Motors by market cap.

**Exam essentials:** The CFA exam loves asking which index type is affected by stock splits (only price-weighted), which has the highest turnover (equal-weighted), and which is biased toward mega-caps (market-cap weighted).

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What are the main equity index construction methods and how do they differ?

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Stock	Price	Shares (M)	Mkt Cap ($B)	Price Wt	Cap Wt	Equal Wt
Vantage Energy	$150	200	$30	50.0%	42.9%	33.3%
Sterling Motors	$100	400	$40	33.3%	57.1%	33.3%
Apex Retail	$50	100	$5	16.7%	7.1% (Note: actually recalc -> total $75B)	33.3%