The application of machine learning (ML) to solve
practical problems is complex. Only recently, due to the
increased promise of ML in solving real problems and the
experienced difficulty of their use, has this issue started
to attract attention. This difficulty arises from the complexity
of learning problems and the large variety of available
techniques. In order to understand this complexity and
begin to overcome it, it is important to construct a characterization
of learning situations. Building on previous work that
dealt with the practical use of ML, a set of dimensions
is developed, contrasted with another recent proposal,
and illustrated with a project on the development of a
decision-support system for marine propeller design. The
general research opportunities that emerge from the development
of the dimensions are discussed. Leading toward working
systems, a simple model is presented for setting priorities
in research and in selecting learning tasks within large
projects. Central to the development of the concepts discussed
in this paper is their use in future projects and the recording
of their successes, limitations, and failures.