Most testing in education happens soon after the instruction. In college, a course of 10 or 15 weeks, ends in an examination of the instruction and assignments made in those previous weeks. For most people, that seems to be the time of maximum retention. A year or a decade later, most people probably have forgotten many of the details of the instruction.
A book I enjoyed reading years ago was "The Knowledge Most Worth Having" by Wayne C. Booth. It is written along the same lines of thought that the English philosopher Herbert Spencer used when he asked "What knowledge is of most worth?" in the middle of the 1800's. Many academics seem to feel that the knowledge of most worth is the knowledge they have. We wouldn't expect a professor of history to state that methods of data analysis are the most valuable knowledge and that knowing the social relations in ancient Mayan culture were the least valuable. A professor who was hired to teach the history of art might well say such knowledge had been of most worth in his life since it formed the basis for his livelihood.
Experts in mental health and the treatment of anxiety and depression might say self-knowledge and acceptance and understanding of one's own emotions is fundamental. Somewhere, the author of the book that formed the basis for "South Pacific" said that it is important to know how to live without being in jail or a mental asylum. That idea is much like Socartes advising to "know thyself".
Many young people express an interest in acquiring whatever knowledge they need to land a good-paying, reasonably pleasant job.
One of several difficulties in choosing what is valuable to teach is that times change and people change. What is valuable for a 20 year old man to learn might be very different from what an 80 year old woman most needs to know.
