  The research area of evolutionary multiobjective optimization (EMO) is
reaching better understandings of the properties and capabilities of EMO
algorithms, and accumulating much evidence of their worth in practical
scenarios. An urgent emerging issue is that the favoured EMO algorithms scale
poorly when problems have many (e.g. five or more) objectives. One of the chief
reasons for this is believed to be that, in many-objective EMO search,
populations are likely to be largely composed of nondominated solutions. In
turn, this means that the commonly-used algorithms cannot distinguish between
these for selective purposes. However, there are methods that can be used
validly to rank points in a nondominated set, and may therefore usefully
underpin selection in EMO search. Here we discuss and compare several such
methods. Our main finding is that simple variants of the often-overlooked
Average Ranking strategy usually outperform other methods tested, covering
problems with 5-20 objectives and differing amounts of inter-objective
correlation.
