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-workdays.” (Wysocki 2012)

“The duration task estimation is a challenging target for the project team. It does not have either fixed rules or clear assumptions to follow, it is related to task resource allocation and could vary from one person to another and from one 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 a case, we can use the finished project’s actual task 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 in the construction field.

2.      Historical data

This is a common method of estimating task duration on construction projects, we recall the actual productivity rates from completed projects to estimate a new project task duration. We use this method to estimate 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 with the duration. In my current project, we have an auditorium hall where the walls and the ceiling are covered with wood panels at different angles, and 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. As a result, we accurately estimated the task duration.

4.      Delphi technique

This method is used when expert advice is not available by asking a group of part participants who are involved in the task about their opinion of task duration. Then tabulate the results and present them 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 opinions to all group members. Then, we ask the group members to write their opinions 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 historical data. We use this method as a planning team in critical activities when we have historical data from more than one project for the same activity.

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I would define the duration types as the following:

Optimistic (O): Best-case scenario.

Pessimistic (P): Worst-case scenario.

Most likely (M): Usual scenario.

The famous formula for estimating the task duration is (O + 4M + P) / 6.

6.      Wide-band Delphi technique

This method is a combination of the Delphi Technique and the 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 the Three-Point formula.

Although (Wysocki 2012) defined six methods of task estimation, I have another case where it cannot fit any of them. 

Resource-driving tasks most probably have not happened before to refer to similarity to other activities. They are unique; therefore, the historical data is not useful. Have a certain way of calculating the duration; therefore, “Expert advice”, the “Delphi technique”, the “Three-point technique”, and the “Wide-band Delphi technique” are not applicable. This type of task depends 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).

Reference:

WYSOCKI, R.K., 2012. Effective project management. 6th ed. Indianapolis, IN: Wiley.

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