Total Market Index Methodology

Market-Capitalisation Weighting

All major total market indices use market-capitalisation weighting — each company's weight in the index is proportional to its total market value relative to the sum of all companies in the index.

How the weight is calculated

The formula is straightforward:

A concrete example: if a company has a market capitalisation of $3 trillion, and the total market cap of all stocks in the index is $50 trillion, that company receives a weight of 6% in the index (3 ÷ 50 = 0.06).

Implications of market-cap weighting

The largest companies receive the largest weights. In a US total market index, the top 10 holdings typically account for 25–30% of the entire index by value, even though they represent less than 0.25% of the total number of holdings by count.

This concentration is an accurate reflection of economic reality — the 10 largest US companies are worth more in aggregate than thousands of smaller ones combined. Market-cap weighting ensures the index mirrors the actual distribution of wealth in the publicly traded market, rather than treating a $10 billion company the same as a $10 million one.

The alternative to market-cap weighting is equal weighting, where every company receives the same allocation. Equal-weighted total market indices do exist but are less common, because they require frequent rebalancing (every company must be adjusted as prices change) and implicitly overweight small companies relative to their economic significance.

Free-Float Adjustment

Modern total market indices do not weight by total market capitalisation — they weight by free-float market capitalisation. The distinction matters.

Total market cap = (all shares outstanding) × (current share price)

Free-float market cap = (freely tradeable shares only) × (current share price)

Shares held by the following are typically excluded from the float:

• Company founders and executive insiders subject to holding restrictions
• Governments and sovereign entities (for state-affiliated companies)
• Strategic corporate investors holding for control purposes
• Employee share ownership plans with lock-up provisions
• Treasury shares (shares held by the company itself)

The rationale is practical: an investor cannot buy shares that are not available for sale. Weighting by freely available shares gives a more accurate picture of what the market can actually invest in.

Impact in practice

For most large US public companies, free-float is close to 100% — the majority of shares trade freely. The adjustment has a more significant effect on:

• Recently IPO'd companies — often 70–80% of shares are held by insiders under lock-up
• Family-controlled businesses — a small free float can make a large company a smaller index weight than its total market cap would suggest
• International indices — government ownership is common in many non-US markets

Eligibility Criteria

A total market index does not hold literally every publicly traded stock. Minimum eligibility thresholds filter out companies that are too small, too illiquid, or otherwise unsuitable for broad index inclusion. These thresholds vary by index provider but typically include:

Minimum market capitalisation

All major total market indices impose a minimum size threshold. The CRSP US Total Market Index covers stocks from large-cap down through micro-cap, with the floor set at a market cap band that captures approximately 100% of the investable US equity universe. Very small companies below the minimum threshold — true nano-caps — are excluded.

Listing requirements

Securities must be listed on eligible US exchanges (NYSE, Nasdaq, NYSE American). Over-the-counter (OTC) bulletin board stocks, pink sheet securities, and other unlisted shares are typically excluded from total market indices, regardless of market cap.

Liquidity requirements

Sufficient trading volume is required for a stock to be included. The minimum liquidity threshold ensures that an index fund can actually execute the trades required to track the index without incurring prohibitive market impact costs.

Security type restrictions

Not all securities are eligible. Standard total market indices exclude: preferred shares, convertible notes, exchange-traded products (ETFs), closed-end funds, and limited partnerships. They typically include common shares and some other equity-equivalent securities, subject to exchange listing and liquidity requirements.

Reconstitution and Rebalancing

A total market index must be periodically updated to reflect changes in the market: new companies going public, existing companies being acquired or delisted, and changes in size that affect index eligibility. This process is called reconstitution.

Reconstitution schedules by index

Reconstitution dates may vary. Refer to each provider's methodology documentation for current schedules.

What happens during reconstitution

At each reconstitution, the index provider evaluates all currently listed securities against eligibility criteria. Companies that have been acquired, delisted, or fallen below minimum thresholds are removed. New companies that have met eligibility requirements since the last reconstitution are added.

Index funds tracking the index must then adjust their holdings to match the updated composition — selling securities removed from the index and buying newly added ones. This trading activity is visible in the market, particularly around the Russell reconstitution (annual, June) which involves hundreds of stocks simultaneously and generates substantial trading volume.

IPO Inclusion Rules

When a company goes public through an initial public offering (IPO), it is not immediately added to most total market indices. A waiting period applies:

• CRSP: New securities are eligible after meeting minimum market cap and liquidity requirements. They are typically added at the next scheduled quarterly reconstitution, though very large IPOs may trigger an expedited addition.
• Russell 3000: IPOs are generally not added until the annual June reconstitution. However, "breakout" IPOs meeting specific size thresholds may be added between reconstitutions under Russell's IPO interim additions rules.
• Wilshire 5000: New securities meeting eligibility criteria are added monthly — one of the more responsive reconstitution schedules, reflecting the Wilshire methodology's emphasis on capturing the complete market promptly.

The waiting period serves a practical purpose: very new public companies often exhibit high volatility and limited trading history. The seasoning period allows the market to establish a stable price before the stock is added to indices that billions of dollars of fund assets must track.

Delisting and Corporate Events

Delistings

When a company's stock is delisted from an eligible exchange — due to bankruptcy, failure to meet exchange standards, or voluntary withdrawal — it is typically removed from the index at or shortly after delisting. Index funds tracking the index sell the delisted security (often at a substantial loss) and use the proceeds to buy other index constituents.

Mergers and acquisitions

When one publicly traded company acquires another in a cash deal, the acquired company is removed from the index at closing and the cash proceeds are redistributed across remaining holdings. In an all-stock merger, the acquiring company's index weight increases to reflect the combined entity; the acquired company is removed.

Spin-offs

When a company spins off a subsidiary as a new public company, the spin-off is typically added to the index at its first reconstitution after listing, provided it meets eligibility requirements.

Why Different Total Market Indices Have Different Security Counts

A common source of confusion: if all total market indices cover "the total market," why does the CRSP US Total Market Index hold ~4,000 stocks, the Russell 3000 hold exactly 3,000, and the MSCI USA IMI hold ~2,400?

The answer lies in three differences:

1. Minimum capitalisation thresholds. Each index sets its own floor. CRSP and Wilshire extend down further into micro-cap territory than the MSCI IMI, which caps at approximately 99% of market cap coverage and excludes the smallest investable stocks.
2. Reconstitution timing. The Russell 3000 has a fixed count of exactly 3,000. CRSP and Wilshire allow the count to float with the market — if more stocks meet their criteria, more are included. The Russell's fixed count means that as companies IPO, others must be dropped at reconstitution to maintain the 3,000 limit.
3. Stock counting conventions. Some indices count multiple share classes of the same company as separate securities; others count the company once. A company with Class A and Class B shares may count as one holding or two depending on the index.

None of these differences translates to meaningfully different investment exposure — all are capturing the same broad US equity market, with differences in composition at the small-cap margins. The large-cap and mid-cap holdings, which represent the vast majority of index value, are substantially identical across all major total market indices.