> For the complete documentation index, see [llms.txt](https://docs.fiinpro.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fiinpro.com/english/bonds/methodology-bond-indices/index-groups.md).

# Index Groups

**1. Reference Price**\
The reference price is applied when there is no transaction on the reference date. It is typically based on the most recent trading day.

In FiinPro-X, the **Clean Price** is used as the reference price. While this may differ from other platforms that use the Dirty Price, applying the Clean Price provides a more accurate reflection.

By definition, the **Dirty Price** of a bond increases over time because accrued interest is added daily. If the Dirty Price were used as the reference, the bond’s yield to maturity (YTM) would appear to rise passively during non-trading periods, which does not accurately reflect market conditions.