a. There is a broader universe of bonds over stocks, and this universe keeps changing as there are new issues, calls, and maturities. The duration and price volatility of bonds are also constantly changing.
b. There are different types of indices for fixed-income securities. These indices differ on the basis of difference in types of:
i. Issuer (the issuers could be government, government agencies, corporations, etc.)
ii. Types of financing (whether it is a general obligation, collateralized, etc.)
iii. Currency of payments,
iv. Maturity,
v. The credit quality of the bond (as ranked by the rating agencies or otherwise),
vi. Whether it provides inflation protection or not, etc.
c. These indices can be categorized as:
i. Aggregate or broad market indices,
ii. Market sector indices,
iii. Style indices,
iv. Economic sector indices,
v. Specialized indices (eg. High-yield, inflation-linked, emerging market, etc.)