Because central bank portfolios hold assets denominated in multiple foreign currencies (such as USD, EUR, and JPY), changes in global exchange rates can cause significant volatility in the reported value of national reserves.

To control this volatility, the board establishes a Benchmark Portfolio—a model asset allocation that reflects the bank’s long-term risk-return preferences. The performance of the active investment desk is measured against this benchmark using the Tracking Error metric:

Tracking_Error = Standard_Deviation(Active_Portfolio_Return - Benchmark_Portfolio_Return)

The risk function sets a hard tracking error ceiling (e.g., 50 basis points annually). This constraint allows investment managers small tactical freedoms to chase yield while ensuring the overall portfolio does not deviate significantly from the approved safety parameters.