TECHNIQUES FOR ESTIMATING TASK DURATIONS IN PROJECT MANAGEMENT As part of planning a project, tasks are assigned to project team members. Durations for task completion are also specified. Task duration estimation is necessary since it helps in cost and resource estimation. Iranmanesh and Mokhtari (2008), further affirmed the importance of project duration estimation by saying that, estimation of time and cost to complete work in a project and follow up them during implementation are major contributors to success or failure of a project, and hence very salient for the Project Manager. According to Hill, Thomas and Allen (2000), developing accurate estimates of overall project duration and its separate activities, is critical in monitoring and controlling of project.
Task estimation is considered as a random variable. Therefore estimating duration of task is an informed or educated guess based on past experience of similar tasks. For complex project, it is very difficult to estimate task duration. Therefore, to make a meaningful task duration estimates, some basic information is essential. This information includes scope document, work breakdown structure (WBS), associated risk, task list, task details, and task dependencies. Schedule risks and the critical path are also essential. Hill, Thomas and Allen (2000) asserted that, the effectiveness of project management techniques depend heavily on the accuracy of the task duration estimates. Thomas (2010) cited in Wysocki (2009) identified six techniques of task duration estimation. These are as followed: similarity to other activities, historical data, expert advice, Delphi technique, Wide-band Delphi technique, and Three-point technique. He identified the Delphi technique and the Three point techniques as the suitable techniques widely use by project management practitioners. My intention is to discuss these methods and how they were applied in my own experience in managing projects.
Historical data: This technique is used in some of our projects. For example, in a National Forest Plantation Development Project, duration of the following activities per hectare per man day have been established based on past records on the implementation of similar projects. The various activities and duration established include: Land clearing – 20 man days
Lopping and debris collection-10man days
Peg cutting-4man days
Beating up-1man day (stumps as planting), 2man days (potted seedlings as planting materials) Maintenance - 20 man days.
The actual duration of some of the activities may vary slightly from past established durations. Seeking advice from experts: Experimental results by Jorgensen (1995) cited in Hill et al (2000) asserted that expert judgment may be very accurate. However, its weakness is the failure to give an objective and quantitative analysis of what factors affect the duration. Also, it becomes difficult to separate real experience from the expert's subjective view. In order to overcome the shortcomings of expert advice, Goodman (1999) and Shepperd (1995) suggested the use of Delphi technique that make use of a consensus arrived by a group of experts. According to Kerzner (1995) and Iranmanesh (2008), the Delphi technique could also be used to set the parameters for a Programme Evaluation and Review Technique (PERT) analysis, whereby the optimistic duration; the pessimistic duration; and the most likely duration for each task are used to arrive at estimated duration.
In my own experience in the implementation of forest plantation project in the Upper West Region of Ghana, we needed to seek expert advice from the Forest Service Division of Ghana in some aspects of the project. For example, we seek advice on the use of the Geographic Positioning System (GPS) to estimate areas of proposed plantation sites. This resulted in better area determination than an earlier manual estimate with a tape measure. Another...