Solar wind-magnetosphere coupling functions have been in use for almost
50 years. In that time, a very large number of formulations have been
proposed. As they become increasingly subsumed into systems analysis and
machine-learning studies of the magnetosphere, it is timely to establish
best practice in their derivation and their limitations. This paper
carried out a number of studies to establish some key points. Particular
attention is paid to the best metric used to evaluate their performance
and how it depends on the application for which the coupling function is
intended.