The reference directions determine the diversity in the selection of solutions from the splitting front, so these directions are typically chosen to be well distributed over the unit simplex. Traditionally the reference direction generation method of Das and Dennis has been used for NSGA-III. Unfortunately, due to the highly structured (combinatorial) nature of the Das-Dennis reference direction generating procedure 42 , the method cannot produce an arbitrary number of directions. In addition, it has been shown that most of the reference directions generated by the Das-Dennis method cross through the boundaries of the unit simplex rather than the interior , inducing a bias in the selection of solutions from the splitting front.