Tariffs or payments are often specified by formulae, as described in Section 3.4.3: Adjustment Mechanisms, to allow regular adjustments for factors such as inflation. The PPP contract could also build in mechanisms for reviewing these formulae-whether periodic, or one-off changes in extraordinary circumstances (with specified triggers). Since these processes are analogous to regulatory tariff reviews, regulatory guidance material may be useful. The World Bank's body of knowledge on infrastructure regulation [#288] section on price level regulation describes key issues in tariff regulation, and guides readers in accessing a wide range of references.