Contingency handling in project planning

The aim of the research described was to investigate empirically the feasibility of an expert planning system which could take account of disjunctive tasks within the plan. This aim was realised which can be added to any appropriate basic planning system. In a conventional procedural not representation of a plan, E.D. Sacerdot (1977), the actions are represented as nodes in the net and ordering relations are shown as arcs joining the nodes. The important point is that ALL the actions must be executed for the plan to succeed. In a real world situation, however, it is desirable for a plan to be able to represent an action which could be carried out in two or more ways. To represent this in the procedural net would require the addition of some action nodes which would not be executed. The term disjunctive planning is used for a scenario such as that described above. A disjunctive plan may include a number of tasks (disjunctive tasks), each of which can be accomplished in a variety of ways. Each of these ways must be able to be fully specified in advance. A large number of feasible plans will thus be represented by this single disjunctive plan description. The number of such plans will increase explosively with the number of disjunctive tasks. It is therefore essential to develop a method for representing this large set of plans without explicitly presenting each one separately