I’ve finished so I’ll start

A critical look at critical Path Analysis

John Bolton, Programmes Director, Parallel Project Training

For a large number of years students of project management have been taught to undertake precedence diagramming as a mechanism for determining the end date of a project and as a guide to the optimisation of resource usage. It is probably the most recognisable aspect of modern project management techniques, culminating as it does in the production of the ubiquitous Gantt cart. It is enshrined in the deepest darkest fundamentals of planning software systems and as such has become widespread it its application, albeit often invisible to the person driving the software. This sometimes blind devotion to software as a one stop shop leads the unwary into the production of poor plans, not fit for purpose. Whilst PDM is a perfectly viable technique there are other variations on a theme using other methods and techniques such as agile and critical chain etc. that are dismissed (knowingly or unknowingly) in the belief that PDM is all you need. These other techniques for manipulating plans have a lot to offer as supplementary mechanisms or even as wholesale alternatives to achieving the best result. The need to consider some other form of planning rigour is further substantiated due to the inherent challenges when deploying the PDM. In this article we look at the limitations of it and of critical path analysis with practical tips on how these challenges can be overcome.

The principles

The theory of PDM is well understood by all project managers and requires that an analysis of the lowest level of work package or activity be undertaken and that they become connected together as in Figure 1.

Figure 1 A simple precedence diagram

The normal relationship between the ‘nodes’ in the network are those of finish to start. This means that any subsequent tasks can only start once the predecessor has finished. The subsequent task does not have to start immediately after the predecessor but merely that it can. Any gap between the predecessor and successor is termed float and can appear either after the predecessor (free float) or before it (independent float). Once the whole network has been analysed the critical path will be determined as the longest path through the network thus determining the end date. The network above has an end date of period 9. The use of this simple approach has a number of limitations and challenges when trying to predict plans for projects. Practitioners need to be aware of these challenges so they do not compromise the plan and so that the project can perform effectively. These challenges are illustrated below.