Chapter 8: Indices and Benchmarking — Part 3: Bond Market Indices and Hybrid Benchmarks

8.6 Bond Market Indices (Debt Indices)

While stock market indices occupy the vast majority of financial media coverage and investor attention, bond market indices are equally essential to the global financial architecture. Investors often exhibit a limited understanding of the growing number of fixed-income benchmarks. However, bond indices serve the exact same critical functions for debt markets that equity indices perform for stock markets—acting as market barometers, benchmarks for active managers, model portfolios for passive funds, and tools for measuring systematic risk.

The Unique Challenges of Constructing Bond Indices

Designing and maintaining a bond index is significantly more complex than constructing an equity index due to several structural characteristics of fixed-income markets:

1. Vast and Broad Universe: The physical size and breadth of the bond market are much larger than the equity market.
2. Multiple Securities per Issuer: In equity markets, a corporation typically has only one class of common stock outstanding (or occasionally a second class of DVR shares). In contrast, a single government or corporate entity can have dozens or even hundreds of distinct bond issuances outstanding at any given time. Each issuance may feature a different coupon rate, maturity date, payment frequency, call/put option, security backing, and credit rating.
3. A Constantly Shifting Population: The constituent list of a bond index changes continuously. New issuances enter the market as entities raise debt capital, while existing bonds mature, are called, or are repurchased, causing them to exit the index automatically. This constant roll-off requires continuous adjustments to keep the index duration and composition stable.
4. Illiquidity and OTC Trading: Most corporate and municipal bonds do not trade on centralized exchanges; instead, they trade Over-the-Counter (OTC). Many debt issues are "buy-and-hold" instruments that rarely trade after their initial primary market placement. This lack of continuous transaction data makes pricing and calculating real-time index values highly challenging compared to exchange-listed equities.

Despite these hurdles, index providers have successfully developed sophisticated rules-based bond indices to track sovereign, corporate, and high-yield debt segments.

8.6.1 Government Securities (G-Sec) Indices

Government Securities Indices are designed to track the performance of sovereign debt instruments issued by the central government. In India, these indices measure the performance of Government of India (GoI) dated securities across various maturity and duration profiles.

NIFTY G-Sec Duration-Based Indices

NSE Indices Limited maintains the NIFTY G-Sec Index Series, which classifies sovereign bonds into six distinct duration buckets based on their Macaulay Duration.

Table 8.3: Sovereign Securities Duration Categories

NIFTY G-Sec Index Construction Methodology

To ensure investability and representative pricing, NSE Indices Limited applies the following systematic rules for its G-Sec indices:

• Eligible Bonds: Within each duration bucket, up to 3 liquid bonds are selected as constituents based on their total trading turnover during the preceding month.
• Minimum Size Filter: The outstanding issuance size of an eligible bond must be at least INR 5,000 crores to ensure sufficient market depth.
• Weighting Scheme: Each constituent bond is assigned a weight proportional to its outstanding issuance amount (value-weighted).
• Rebalancing Frequency: The G-Sec indices are rebalanced and reconstituted on a monthly basis to account for shifting maturity profiles and liquidity.

NIFTY T-Bills Indices

In addition to long-term coupon bonds, index providers maintain dedicated benchmarks for short-term, zero-coupon government debt known as Treasury Bills (T-Bills). The NIFTY T-Bills Index Series consists of five maturity-specific indices and one consolidated aggregate index.

Table 8.4: NIFTY Treasury Bill Indices

T-Bill Index Selection Rules:

• For the 91-day, 182-day, and 1-year indices, the most recently auctioned T-Bill is automatically selected as the sole index constituent.
• For the 30-day and 60-day indices, the outstanding T-Bill whose remaining residual maturity is closest to 30 or 60 days, respectively, is selected.

8.6.2 Corporate Bond Indices

Corporate bond indices are engineered to track the performance of taxable, private-sector debt securities issued by corporates, financial institutions, and Public Sector Undertakings (PSUs).

Because credit risk is a vital pricing factor in corporate debt, these indices are segregated by both Macaulay Duration and Credit Rating segments. The primary rating categories tracked in the Indian market include AAA, AA+, AA, and AA-.

NIFTY AAA and AA Rated Bond Indices

These indices measure corporate debt performance across the six standard Macaulay Duration buckets.

• Issuer Diversity: Each index can consist of up to 14 unique issuers. To maintain diversification, each issuer is represented in the index solely by its most liquid bond issuance.

• Determining Issuer Liquidity: To select the 14 issuers, index providers calculate a proprietary Composite Liquidity Score for all eligible corporate bonds during the previous quarter. This score is computed using a weighted formula:

  Composite Liquidity Score = (Trading Value * 80%) + (Number of Days Traded * 10%) + (Number of Trades * 10%)

NIFTY AA Category Aggregated Indices

These indices measure the combined performance of AA+, AA, and AA- rated corporate bonds across the six duration buckets. They are derived as an aggregate index of the individual sub-category indices, drawing up to 14 of the most liquid issuers from each rating sub-category, making them highly diversified credit benchmarks.

NIFTY A Rated Bond Indices

These indices measure the performance of corporate bonds rated exactly "A". Due to the lower liquidity profile of single-A rated paper, they are tracked across only two Macaulay Duration buckets:

1. Short Duration: Macaulay Duration up to 3 years.
2. Long Duration: Macaulay Duration greater than 3 years.

Key Filters for NIFTY A Indices:

• Minimum Issuance Size: The bond must have an outstanding issuance size of at least INR 50 crores.
• Constituent Selection: Eligible issuers are selected directly from primary market issuances.
• Weighting: Individual bonds are assigned weights based on their total outstanding issuance size.
• Rebalancing: Reconstitution occurs on a monthly basis.

NIFTY Fixed Income Aggregate Indices

For investors seeking a single, all-encompassing debt market benchmark, providers construct aggregate fixed-income indices. The NIFTY Fixed Income Aggregate Series consists of 13 indices that blend various proportions of Government securities, corporate bonds of different rating categories, Commercial Papers (CPs), Certificates of Deposit (CDs), Treasury Bills, and overnight lending rates.

8.6.3 High-Yield and Global Bond Indices

High-Yield Bond Indices

High-yield bonds (commonly referred to as "sub-investment grade" or speculative-grade debt) consist of bonds rated below BBB- (or equivalent). These issuers carry a higher risk of default but offer higher coupon rates to compensate investors. Globally, prominent index providers (such as Bloomberg, ICE Data Indices, and Markit iBoxx) maintain dedicated High-Yield Bond Indices to track this risky segment.

Global Bond Indices

Similar to global equity benchmarks, global fixed-income indices exist to allow institutional investors to track international debt allocations.

• FTSE World Government Bond Index (WGBI): Maintained by FTSE Russell, this is a prominent global index tracking sovereign debt across more than 100 developed and emerging nations, incorporating over 18,000 unique bond issues.
• Indian Context: Historically, Indian government bonds have not been included in major global aggregate bond indices due to capital controls and tax registration requirements. The Indian Government and Reserve Bank of India (RBI) have actively worked with international index providers to resolve these barriers. Index inclusion is highly coveted as it automatically triggers passive, multi-billion-dollar foreign portfolio inflows from global funds tracking these benchmarks.

8.6.5 Total Return Bond Index (TR Bond Index)

Just as with equities, debt indices must be calculated on a total-return basis to reflect the true economic performance of a bond portfolio.

• Price Return Debt Index: Measures only the capital gain or loss of the bonds resulting from market yield fluctuations. It completely ignores coupon payments.
• Total Return Bond Index: Tracks the complete return of holding the index portfolio. This value-weighted return incorporates three distinct sources of cash flow and value:
  1. Price Movements: Capital appreciation or depreciation of the underlying bonds.
  2. Accrued Interest: The interest income that accumulates daily on the bonds between coupon payment dates.
  3. Cash Flows: The physical receipt of interest coupon payments, principal redemptions, or bond repurchases, which are assumed to be immediately reinvested back into the index portfolio.

8.7 Stock-Bond Hybrid (Composite) Indices

Many conservative or balanced investment funds (including certain Category III AIFs) do not invest exclusively in one asset class. Instead, they maintain dynamic or static exposures to both equities and fixed-income instruments. To provide appropriate benchmarks for these multi-asset strategies, index providers construct Composite Hybrid Indices.

The NIFTY Hybrid Index Series

The NIFTY Hybrid Index Series comprises six indices that combine the equity-focused NIFTY 50 Total Return (TR) Index and various NIFTY Fixed Income Aggregate Indices in pre-defined target weight proportions.

Table 8.5: NIFTY Hybrid Index Series Composition

The Rebalancing and Weight-Drift Mechanism

When managing a composite index, index providers must establish rules to handle weight drift. Because equity prices and bond prices move independently, the actual weights of the sub-indices will drift away from their target allocations daily. For example, during a strong equity bull market, the equity portion of a 50:50 index might drift up to 53% within a week.

• Daily Calculation: The index values are calculated using the drifted, market-driven daily weights.
• Monthly Reset: To maintain alignment with the fund's mandate, the weights are programmatically reset back to their pre-defined target levels (e.g., exactly 50% and 50%) on a monthly basis.

Summary of Fixed-Income and Hybrid Index Categories

Important Terms & Definitions

1. Macaulay Duration: A measure of the weighted average time an investor must hold a bond until the present value of its cash flows equals the bond's purchase price. It serves as the primary tool for categorizing fixed-income duration buckets.
2. Composite Liquidity Score: A quantitative score used by NSE Indices to rank corporate bonds. It allocates 80% weight to trading value, 10% to the number of days traded, and 10% to the total number of trades in the preceding quarter.
3. T-Bill Auction: The weekly primary market issuance process through which the Reserve Bank of India sells Central Government Treasury Bills to market participants.
4. Accrued Interest: The interest that accumulates on a bond daily since the last coupon payment date, which must be incorporated into a Total Return Bond Index.
5. Weight Drift: The passive shift in the percentage weight of assets in a hybrid index caused by the unequal performance of the underlying asset classes.

Candidates’ Exam-Relevant Takeaways

• 👉 Bond indices are harder to construct than equity indices because companies issue multiple series of bonds, the universe is much larger, and bonds constantly mature or roll off.
• 👉 The NIFTY G-Sec low duration category covers a Macaulay Duration range of 6 months to 12 months.
• 👉 To be eligible for the NIFTY G-Sec Index, a bond's total outstanding amount must be more than INR 5,000 crores.
• 👉 NIFTY T-Bills Indices are reconstituted weekly on the day of the T-Bill auction.
• 👉 The Corporate Bond Index liquidity score allocates 80% weight to trading value and 10% weight each to the number of days traded and the number of trades.
• 👉 The NIFTY A Bond Index requires a minimum corporate issuance size of INR 50 crores and is rebalanced monthly.
• 👉 A Total Return Bond Index must include price movements, accrued interest, and cash flows such as coupons and redemptions.
• 👉 In the NIFTY Hybrid Index series, weights of the equity and fixed-income components are reset to pre-defined levels on a monthly basis to correct weight drift.