The common attribute of external indicators is that they examine inputs and outputs for a project.
External indicators are expressions of various forms of efficiency, whether the efficiency is related to budgets, water, or crop yields.
But even more than that, they only require knowledge of inputs and outputs to the project. By themselves, external indicators do not provide any insight into what must be done to improve performance or efficiency.
The identification of what actions must be taken to improve these external indicators comes from an examination of internal indicators, which examine the processes and hardware used within the project.