History and Some Latest Developments of Precedence Diagramming Method

Abstract

Although the birth of Precedence Diagramming Method (PDM) is not as radiant as of CPM or PERT, but it is definitely the prevailing scheduling technique of our times. This popularity is due to its modeling flexibility over other techniques and the easy-to-understand mathematical model behind the technique. However, even this technique has its own limitations; modeling overlapping activities in a proper way seems to be a never-ending debate. The reason of this can be found inthe fundamentals of PDM technique; the four precedence relationships that form connections between the end-points of the activities, with constant production speed. These fundamentals of PDM have their own consequences to scheduling practice; it is more and more apparent among professionals that activity overlapping in PDM cannot be modeled adequately. Different solutions were proposed for solving this problem from the application of negative lag, through the combination of SS and FF relations to the fragmentation of activities. All these solutions have their shortcomings. Probably the fragmentation technique has led to the development of point-to-point type of relation that can connect any arbitrary points of the dependent activities. The objective of this paper is to analyze the pros and cons of different solutions that are used for modeling overlapped activities, then to show how newly defined point-to-point relations can be used for this purpose. Algorithms that handles point-to-point relations with minimal and maximal lags are also presented. The main finding of the paper is that newly developed point-topoint relations are better from theoretical and practical point of view than the solutions based on traditional precedence relationships, but they still cannot provide theoretically perfect solution for overlapping. This paper is the fully extended version of a paper building on the results already presented on the Creative Construction Conference [1].