# 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.fiinpro.com/english/bonds/methodology-bond-indices/index-groups.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.