Project portfolio selection problems: Two decades review from 1999 to 2019

Project portfolio selection (PSP) problems have been explored for more than two decades. As a result, in the last two decades, the literature of PSP has experienced a sharp increase. Despite this increase, the literature has not been thoroughly reviewed. The main goal of this paper is to investigate and explain the features, state of the art, research directions, and real-world applications of PSP. To pursue this goal, the paper investigates PSP form aspects of PPS criteria, uncertainty modelling tools, selection and optimization methods, and application areas. More than 105 journal papers have been reviewed. The review has resulted in the categorization of applied tools in PPS in addition to the presentation of new research directions.