The Average Internal Rate of Return (AIRR) approach presented in the article shows that projects are linked to a return function, not just a single rate of return. Any rate of return definition in the literature is a type of AIRR. When choosing between projects, the AIRR acceptability criterion can be applied to the incremental project. The AIRR method easily calculates the rate of return on the initial investment, unlike the Internal Rate of Return (IRR) which only gives the rate of return on the total capital implied by the IRR itself.