The fraction of outstanding bonds accounted for by the three largest issues in the market. Issuer concentration is a measure of market depth; a high percentage implies a highly concentrated market. Less developed bond markets would normally have a high issuer concentration index, indicating a narrow issuer base. In the very early stages of bond market development, bond issuance should be highly concentrated around a few issues in order to enhance liquidity. A low ratio in a developing market might indicate that individual issues are too small to generate trading activity. As markets develop and bond issuance activities become more regular and generalized across the yield curve, issue concentration ratios decrease.