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| History of Project Scheduling |
From Ancient Times to the 19th Century
Even in the ancient times, projects were often constructed with a great consciousness of the importance of time. Consequently, the concept of project scheduling is not new. The pyramids, the Great Wall of China and aqueducts of the Romans are over 2000 years old. None of these constructions and assignments could have been completed without planning and some kind of schedules. Archaeologists have been studying these endeavors, but there is little evidence of formal processes until the 20th century.
Between the 15th and 17th centuries, concepts of engineering science were emerging and these were applied to the management of large projects. In the 18th century, the engineering complexity of projects increased and separation of design and construction works was the birth of consulting. (Morris, 1994) One of the earliest known scheduling tools is Karol Adamiecki’s Harmonygraph.
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The Theory of Work Harmonization was developed in Poland in 1896. The Harmonygraph includes features which make it the widely-used CPM and PERT methods developed 60 years later. The Harmonygraph has a date scale on the vertical axis and a list of tasks on the horizontal axis on the top. The sequence and duration of tasks are shown by a sliding tab like a bar in a bar chart. The Harmonygraph also indicates predecessors and successors of each task. Adamiecki’s chart never became popular in project scheduling because of the unpopular language. The chart was not published until 1931. (Cornish, 2008 and Morris, 1994)
Early 20th Century
- Gantt Charts American engineer Henry Gantt developed a bar chart (Gantt chart) in 1917 as a visual production scheduling tool. It became a popular method and is in wide use today in an essentially unaltered form. The Gantt chart indicates tasks and time in a graphical format allowing the allocation of tasks in time horizon, but not determining interdependencies between tasks. Gantt charts were first used on large construction projects and they proved their efficiency in remarkable projects like the Hoover Dam, started in 1931 (Wikipedia, 2008). Another type of chart which came into regular use in the 1950s was a milestone chart. It was used along with Gantt charts. Major projects were subdivided into components with target dates set for completing tasks required to achieve each milestone. Milestone charts are also widely used today, especially for management reporting. A major benefit is the easy communication of large amounts of information.
Description of Gantt Charts
Gantt charts are the simplest and most widely-used scheduling technique today in all fields because of its easily understandable format. Almost any user can read the schedule without prior training or knowledge. The Gantt chart can be used as an only scheduling tool when preparing a schedule or it can be used as a graphic presentation format for schedules established by other scheduling techniques.
Late 20th Century
- Critical Path and PERT Methods The Critical Path Method (CPM) and Program Evaluation and Review Technique (PERT) were developed simultaneously in the 1950s. They are remarkably similar to each other, both using an arrow diagramming method, but were developed for fundamentally different business fields. CPM was aimed for the construction and maintenance industry where technologies and processes were largely known and estimations of task durations could be done with some accuracy. In contrast to CPM, PERT was focused on military research and development (R&D) where time pressures were high and cost a secondary issue. In an R&D environment, activity durations were difficult to estimate, thus, PERT emphasized probability.
Development of the Critical Path Method
The origin of the CPM may be traced back to 1956. E.I. Du Pont de Nemours (Du Pont) in the USA was one of the very first owners of computers installed in a commercial business and they were looking for useful tasks to do with their UNIVAC I computer. One possible use of the computer was to determine the best trade-off between time and cost and Du Pont’s management felt that estimating of scheduling seemed like a practical task for optimization. By 1957, Morgan Walker assisted by James Kelley had developed a model for the time-cost problem.
They could demonstrate that focusing effort on the right tasks could reduce time without significantly increasing cost instead of recovering lost time with a bulk of labour. The challenge was to determine the right tasks. The activity-on-arrow diagram was used to explain the calculations of the method. Development continued through 1958 and in 1959 CPM was presented publicly. Regardless of many innovations, CPM nearly died as a concept until 1959. However, Kelley together with his associates formed their own consulting firm and commercialized CPM. They focused on schedule (rather than cost) and organized training of the method. Although CPM was expensive – solving scheduling problems could cost the price of a small car – it becomes very popular and moved to the forefront of scheduling methods (overtaking PERT).
Development of PERT and Associated Systems
The PERT method was developed by the US Navy Special Projects Office (SPO). The SPO for a missile program (Polaris) was established in late 1955. During 1956, the SPO investigated the systems used by other companies and organizations for large-scale projects, but could not find significant added value. A small team consisting of SPO members and outside consultants was established to progress the development work. The team developed a list of features for the system, including careful task time estimates, probability distributions, and precise knowledge of the sequencing of tasks.
Core parts of the method included the collection of task estimates from bench engineers and the calculation to identify the longest sequence of events in the project, also called the critical path. In mid-1957 the basic concepts of PERT procedures had been published, and the method was being run on computers. In fact, the PERT method was not widely used within the Polaris program because of lack of trust in the method in SPO. Interestingly the method was effectively publicized because the first Polaris missile was launched in 1960, and by 1964 the PERT bibliography comprised almost 1000 books and articles.
Activity-on-Arrow Method
The linked Gantt chart presented earlier can show precedence relationships between tasks, but is not suitable if the project schedule is complex. Network techniques can handle complex dependencies better. The graphical presentation of schedule can be done in a Gantt chart format. Two types of network representations exist: activity-on-arrow (AOA), and activity-on-node (AON). Activity-on-arrow will be discussed first and the activity-on-node method will be presented later on because both, CPM and PERT, activity-on-arrow methods were eventually replaced by precedence diagramming methods (activity-on-node) in the 1970s.
The AOA technique is also called the Arrow Diagramming Method (ADM). AOA is a method to formulate a schedule network diagram. In order to construct a network, a list of tasks, precedence relationships, and estimations of task durations are needed. Tasks (activities) are presented as arrows which are connected at nodes which present events (circles). The arrows also define the precedence relationships. In Figure 4 in the next section, a network diagram constructed using the AOA method is shown.
Because tasks are linked through nodes, finish-to-start connections are only used. However, it is possible to define all other task relationships if dummy activities are introduced. Dummy activities have no work content and are usually drawn with dashed lines. (PMI, 2004 and Turner, 1999)
Critical Path Method
CPM is a network technique for scheduling a set of project tasks. The critical path is the sequence of project network tasks with the longest overall duration, which determines the shortest completion time of a project. When using CPM, the earliest and latest start and finish dates are calculated for all scheduled tasks. These are determined by performing a forward and a backward pass analysis through the network paths. This process indicates which tasks are critical, in other words, on the longest path. Schedule flexibility is calculated by the difference between early and late dates, and is termed total float.
It shows the tasks which can be delayed without making the project longer. Critical paths have a zero total float, and tasks on a critical path are called critical tasks. Any delay on the critical path directly impacts the project completion date. The critical path can be affected by adjusting activity durations, precedence relationships, leads, and lags. The concept of the critical path is seen as useful because it draws attention to the tasks that need the closest monitoring. (PMI, 2004)
PERT
The PERT method can be considered an extension of CPM by incorporating variability in task duration estimates. The uncertainty of task durations is taken into account by using three-time estimates for each activity. In the PERT the most commonly applied distribution is a beta-distribution, where O and P are optimistic and pessimistic estimates and M the most likely duration. With different O, P, and M values, almost any skewed distribution can be prepared and in most cases, the distribution is skewed to a pessimistic direction, indicating that a delay is more likely than an early completion.
All task durations are considered stochastically independent, which means that a delay in one task will not lead to a similar delay of another activity task. In the PERT method the expected durations are calculated for all tasks and then the network is calculated by the AOA method as in CPM using these expected values as task durations.When using beta-distribution, the expected task duration E(t) and variance Var(t) is calculated: No alt text provided for this image
The variance indicates the uncertainty of the duration. A larger variance indicates greater uncertainty in the estimates.
European Developments The development of scheduling methods and systems did not take place solely in the USA. Europeans also developed different systems for scheduling, but none of them remained as a dominant method. At the end of the 1960s, the PERT and CPM methods had developed as standard dominant systems. The British chemical company Imperial Chemical Industries (ICI) developed a technique which was a predecessor of CPM as early as in 1955. ICI’s “controlled sequence duration” was used for plant maintenance scheduling.
The method was never widely publicized and little information has been found concerning the system. The Central Electricity Generating Board (CEGB) in the UK was developing their version of critical path and in 1957 CEGB came up with the technique of “longest irreducible sequence of events” (later renamed “major sequence”) in the maintenance of generating plants. With this scheduling technique, CEGB was able to reduce the shutdown time of plants by over 40%. This method was not publicized and never became a popular system. A system which was like PERT called Setevoe planirovanie i upravlenie (network planning and management) was developed in the Soviet Union (Russia).
It was published in 1969, but was never popular in Western countries. The Metra Potential Method (MPM) was developed in 1958 in France. MPM introduced the idea of lags in the scheduling algorithms. A method called RPS (Regeltechnischen Planning und Steuerung) was developed in Germany in 1960 which also uses lags in the AON system to calculate critical paths. Although these systems were developed in Europe, John Fondahl’s precedence diagramming method, published in 1961 in the USA, has many similarities to these systems.
The mid-1960s Onwards - Growing Construction Industry
Until the mid-1960s the evolution of modern project management was virtually dominated by the US defense and aerospace sector in their major projects (Manhattan atomic bomb, Polaris missile, and Apollo lunar), since almost all the basic techniques and methods were pioneered there. After that, the number of projects in the construction industry using modern project management methods increased strongly. To overcome technical and organizational challenges, new insights and methods were developed for project management. In addition to construction industry developments, interest towards project management was also growing in business schools, academia, and general industry.
That development broadened and expanded project management to other fields and increased research efforts. During the 1960s and 1970s network scheduling, cost control and other project management tools had been implemented throughout the USA and Europe. Hundreds of articles were published only about network scheduling. Heuristic methods were developed to deal with resource allocation, which was largely a theoretical field at the end of the 1960s. The complexity of resourcing required unrealistic amounts of mainframe computer time, which was very expensive.
The late 1970s to the Present Day - Computer Systems
Through the early 1980s, project schedules were usually prepared by first drawing them manually to sort out problems. Then the corrected and checked schedules were loaded in the mainframe or mini-computer and calculated. On mainframe days, it was considered that inexperience of scheduling could cause costly and time-consuming problems. To prevent this, schedulers were trained through a process of practical training and mentoring. That led to the evolution of skilled project schedulers who possessed the expertise of scheduling.
Standardization of scheduling processes and collection and use of schedule data was developed to be more effective in organizations with departments for scheduling. During the late 1970s, the arrival of more powerful project scheduling systems running on micro-computers caused the major change in scheduling. Soon after computers software tools for scheduling also appeared. Planning Services in the UK launched the first commercial scheduling software for Apple computers in 1979. The first IBM personal computer (PC) was introduced in 1981 and a year later, a scheduling software was launched for PC systems. Primavera and Microsoft have been competing since the early days of PC-based scheduling software tools. Primavera was founded in 1983 and it was focusing on the then mainstream Disk Operating System (DOS).
Microsoft was founded in 1975 and the first version of Microsoft Project was released for DOS in 1984 by a company working for Microsoft. That first version introduced the concept of dependency lines between tasks in the Gantt chart. The first Windows-based version was released in 1990. (Wikipedia, 2008) In the latter half of the 1980s the number of relatively cheap and easy-to-use PC- based scheduling tools increased greatly. The low-end tools made scheduling systems available to many users and allowed everyone to do cheap computer-based scheduling.
This had two significant impacts: by the early 1990s no one was preparing schedules manually anymore, and the number of non-professional users formulating schedules on a part-time grew substantially. Interestingly, attention has currently refocused on the role of professional schedulers because of growing interest in enterprise-level systems. A number of available Internet-based scheduling tools and niche systems have found market potential. Different kinds of risk analysis, simulation tools and add-on packages for dominant systems such as Microsoft Project and Primavera are increasing.
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The Author: Dariusz Wolejszo
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Project Control specialist offering more than 15 years of leadership in design, construction, project development, and commissioning of high-profile oil, gas, petrochemical and power facilities. Strong experience in a project controls management role, working for international construction companies.
Great understanding of Planning, Cost, Estimating, and document control. Excellent communication and interpersonal skills. Team player, self-driven and good in high pressure dynamic situations. People and result orientated individual with strong understanding the motivational requirements whilst working in projected organizations.
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Oil & Gas (Onshore & Offshore), Power Plants, Shipyards, Railway – ERTMS
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