How are fixed income indices constructed?

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AcadiFi · Accepted Answer

Fixed income index construction differs fundamentally from equity because the universe is vastly larger (tens of thousands of outstanding bonds versus thousands of stocks), bonds mature and are redeemed, new issuance is continuous, and liquidity varies enormously across issues.

Core design choices: (1) universe definition (investment grade only, high yield, global, emerging market); (2) inclusion rules (minimum outstanding size such as $300M for Bloomberg Global Aggregate, minimum maturity, credit rating thresholds); (3) weighting scheme (market-value weighted is dominant but inherits the 'more-debt-heavier-weight' paradox); (4) rebalancing frequency (monthly for most flagship indices); and (5) pricing methodology (matrix pricing, evaluated prices from third parties like ICE Data Services because many bonds do not trade daily).

For Meridian-style aggregate indices, market-value weighting means the largest debtors (governments, financials) dominate. This creates the 'bums problem': indices overweight the most indebted issuers, which are not necessarily the most creditworthy. Alternative schemes like GDP-weighted sovereign indices, fundamental weighting by capacity-to-pay metrics, and equal weighting address this but have their own trade-offs.

Rebalancing events include monthly adds (new issuance meeting criteria), drops (bonds within one year of maturity, downgrades below investment grade), and adjustments for called/tendered bonds. Index providers publish a preliminary month-end basket so managers can position.

For passive managers replicating a bond index, full replication is usually infeasible due to liquidity and issue count, so representative sampling or stratified sampling by duration, credit, and sector bucket is standard.

How are fixed income indices constructed?

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