An UDE is a fact or an entity from reality that explains why it is so difficult to achieve the higher level of performance measurements.

When there is a gap in performance it is a signal to search for UDEs.

The UDE is an ‘effect’ and its existence is indisputable. It is ’undesirable’ because
it endangers/reduces/prohibits achieving a valid need, objective or the goal of a system.

The 10 UDE Rules:

1. UDE wording consists of a short, simple sentence – it cannot contain a cause-effect relationship.
2. An UDE is an ongoing problem that exists in your reality, and which does not allow your system to perform better.
3. It is a not a one-time recent action or event – its wording cannot have verbs like ‘get,’ ‘go,’ etc.; the wording should describe the state of the system.
4. It is within your area of responsibility.
5. Something can be done about it.
6. It must not be blame.
7. It must not be a speculated cause.
8. It must not be a hidden solution (wishful thinking for solving the problem).
9. It does not require a clarification of its negative impact/effect.
10. It must not be subjective – it should not contain such words as ‘difficult,’ “hard,” “poor,” etc.

Sources:
Oded Cohen and Jelena Fedurko, Theory of Constraints Fundamentals, 2012; 
Jelena Fedurko, Through Clouds to Solutions: Working with UDEs and UDE Clouds, 2013