Methods to estimate task and activity duration
“The duration of a project is the elapsed time in business working days, not including weekends, holidays, or other non-work days. (Wysocki 2012)
The duration task estimation is a challenge target to the project team. It does not has neither fixed rules nor clear assumptions to follow, it is related to task resources allocation, and could vary from person to another and from project to another. (Wysocki 2012)
(Wysocki 2012) identified six methods to estimate the project duration as per the following:
1- Similarity to other activities
This method is applicable when we have two similar projects, one finished and another ongoing. In such case, we can use the finished project actual tasks duration along with lessons learned to estimate the new project duration. This method is rarely used in construction projects; it is very hard to find two typical projects on construction field.
2- Historical data
This is a common method of estimating tasks duration on construction projects, we recall the actual productivity rates from completed projects to estimate a new project tasks duration. We use this method to estimate the traditional activities such as masonry works, plaster works, painting, flooring … etc.
3- Expert advice
whenever we have a new task never executed before, or require a special arrangement, we usually refer to an expert to help us estimating the duration. In my current project, we have an auditorium hall where the walls and the celling are covered with wood panels in different angles shapes, the installation method statement of the wood and the hanging system was not clear to the planning team. Therefore, we called the wood subcontractor in the planning process to discuss with him the fabrication, hanging system, and wood installation method statement and the forecast productivity rates. In result, we accurately estimated the task duration.
4- Delphi technique
This method is used when the expert advice is not available by asking a group of participation who are involved on the task about their opinion of task duration. Then tabulate the results and present to the group in a histogram labeled First Pass. We then identify the highest and lowest results and ask the group members who wrote these values to explain their opinion to all group members. Then we ask the group members to write their opinion again to present the Second Pass histogram. Again, we do the same as we did in the First Pass. Finally, we take the average results of the Third Pass as the group’s estimate.
5- Three-point technique
“Task duration is a random variable. If it were possible to repeat the task several times under identical circumstances, duration times would vary.” (Wysocki 2012)
This is the best way of estimating a task duration when we use it with a historical data. We use this method as a planning team in the critical activities when we have a historical data from more than a project for the same activity.
I would define the duration types as per the following:
Optimistic (O): Best-case scenario.
Pessimistic (P): Worst-case scenario.
Most likely (M): Usual scenario.
The famous formula of estimating the task duration is (O + 4M + P) / 6 .
6- Wide-band Delphi technique
This method is a combination of Delphi Technique and Three-Point Technique. Instead of asking the group members to write one number for the expected task duration, they need to guess three numbers (Optimistic, Pessimistic and Most likely). In addition, the histogram calculation takes the average as per Three-Point formula.
Although, (Wysocki 2012) defined six methods of tasks estimation, but I have another case where it cannot fit to any of them. Resource driving tasks most probably not happened before to refer to Similarity to other activities. Are unique, therefore the Historical data is not useful. Have a certain way of calculating the duration; therefore, Expert advice, Delphi technique, Three-point technique and Wide-band Delphi technique are not applicable. This type of tasks depend purely on the productivity rate of the resources.
Example: In my current project, the only way to erect the mega roof steel and glass panels is by using the available tower cranes. To estimate the duration required to finish this task we rely on the tower crane productivity, nothing else.
Task duration = (Panels Quantity) / (Tower crane productivity per day * number of tower cranes)
WYSOCKI, R.K., 2012. Effective project management. 6th edn. Indianapolis, IN: Wiley.
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I’m planning a new oil & gas project but I don’t have any reference in assigning duration to each activity, civil, mechanical, etc. Could you advise what reference should I refer to?
Usually I consultant the project manger and construction manager for the activities I cannot estimate it’s duration. Furthermore, for activities in special categories, we might ask the specialist of the category to advise you about the most likely duration.