Expert knowledge is highly idiosyncratic:
- Build stair steps 7 inches high and 10 inches wide.
- Two times height plus width should equal 25 inches.
- Width times height should equal 72 inches.[Parker, T., Rules
of Thumb, Boston, MA: Houghton Mifflin Publishers, 1983.]
- Different rules may be generated for the same phenomena.
- The rules may have no fundamental validity
and may give bad answers outside a limited domain of applicability.
- The rules generally work within the limits of
applicability, but the expert often doesn't
know what the limits are.