In a hierarchy every employee tends to rise to his level of incompetence.
Peter's Corollary: In time, every post tends to be occupied by an employee who is incompetent to carry out its duties.
Peter's Corollary states that incompetence spreads over the organization since "in time, every position tends to be occupied by an employee who is incompetent to carry out his duties" and adds that "work is accomplished by those employees who have not yet reached their level of incompetence". (The Peter Principle Revisited: A Computational Study)
The Peter Principle - Why things Always Go Wrong, Profile Books
The Vardi Amendment: "But when managers reach their level of incompetence, they are simply promoted to a higher level in a different organization, as this is the easiest way to get rid of them."
Ronald E. Riggio: Do the Incompetent Rise to the Top? Peter Principle Revisited, PsychologyToday, 2010-04-05
Research: Do People Really Get Promoted to Their Level of Incompetence?, HBR, 2018-03-08
James Ike Schaap: The Peter Principle: Is This Forty-Year-Old Universal Phenomenon in Decline or Growing? (PDF)
Organizations: A Glossary of Incompetence, Time, 1969-03-28
How & Why We Fail Upwards: Revisiting the Peter Principle, Compensation Force, 2009-04-03
What Is the Peter Principle? Rising to Incompetence, Shortform, ????-??-??
The Peter Principle: How to Overcome the Perils of Promotions, the Mind Collection, ????-??-??