Legacy rules hamper fixed-income index construction

Exchange-traded funds for fixed income now trade like stocks and hold trillions of dollars in assets, but many broad bond indexes continue to use methodology rules developed for a 1990s over-the-counter market. Those legacy rules include liquidity screens, minimum issue sizes and sampling conventions.

When broad bond benchmarks were created, many corporate and sovereign bonds traded rarely and price discovery was limited. Index providers applied filters to keep benchmarks tied to securities that could be located and traded, and they sampled an issuer’s debt instead of listing every issue. Those choices reduced implementation complexity in a market where full replication was impractical.

The ETF creation and redemption mechanism and wider electronic trading have changed how bond liquidity forms. Authorized participants assemble and break apart baskets of thousands of bonds daily, and more individual issues now print prices regularly. Securities that once traded infrequently now have clearer and more continuous price signals.

Despite those market changes, many index methodologies retain legacy screens and sampling rules. Funds that track those benchmarks must operate within the universe the index defines, which affects which bonds managers can hold and how they implement exposure.

Sampling assigns the selection of specific bonds to index providers rather than listing every eligible security. As a result, investors tracking a sampled index are exposed to a curated subset of an issuer’s debt. An index that lists all eligible bonds would provide portfolio managers with the full opportunity set and allow them to decide which issues to buy based on trading costs and execution tools.

Index construction is increasingly assessed for operational and market-structure fit, including how rules behave during rebalancing, how they interact with live trading and how they perform under market stress. Those operational considerations influence how closely a benchmark reflects the investable market and how straightforward it is for ETF providers and managers to implement an index-tracking strategy.

Another longstanding convention is weighting by total outstanding debt, which increases an issuer’s benchmark weight as it borrows more. Index providers and asset managers must choose whether to preserve historical methodology elements or to redesign benchmarks to reflect current trading patterns. Changes to methodology could alter which securities are included, the role managers play in implementation and the risk and concentration profiles of passive fixed-income funds.