(5)High-quality reference data, including data provided by consolidated tape providers, where available, should provide a reliable and accurate presentation of execution prices in the market. Investment firms should use such data when selecting execution venues and when monitoring, on a regular basis, the effectiveness of their order execution policy. Investment firms should be allowed to use alternative external data sources where data from such sources are reliable and accurate. Investment firms should make sure that the reference data they use contain the data from the most liquid execution venues, including data from the execution venues that are the most relevant markets in terms of liquidity for the instruments traded within each class of financial instruments.