Lessons learned

This overview by Michael Quinn Patton and Ricardo A. Millett provides a definition and discussion of how lessons learned can be derived and from what sources. 

Excerpt

"The ill-defined, trivial, and over-generalized ways in which the phrase “lessons learned” has come to be used challenges evaluators to distinguish between casual/informal notions of lessons learned and what we call “high quality” lessons learned. High-quality lessons learned contain knowledge that can be applied to future action and can be derived from screening according to specific criteria"