Estimating Task Duration in Project Management

If you want to make your project management robust, estimating the task duration is of utmost importance. In this article, we will discuss how to estimate task duration and will see some practical examples.

Content

• What is task duration in project management?
• How to Determine the Task Duration?
  • The Three-point technique (with historical data)
  • The Delphi method: Get the benefit from experienced people
• Practical Examples of estimating the task duration
  • The Truss Lifting Task: The resource constraint method
  • Estimating the Duration of a Slab Formwork Removal
  • Considering Space Constraint While Estimating Duration of a Task
• The Best Principle

What is task duration in project management?

In project management terms, task duration is the actual time it takes to complete.

Planned duration of a task = Planned finish date – Planned start date

Actual duration of a task = Actual finish date – Actual start date

As we are aware, Tasks are the smallest unit of a project. So finding an easy and correct way to estimate their duration is of utmost importance to ultimately find out the project duration.

How to Determine the Task Duration?

There are 3 fundamental steps in preparing the most basic project schedule. They are:

1. Identifying the tasks involved in a project
2. Setting the inter-task relationships
3. Estimating the duration of each task and assigning the same to them

While preparing a project schedule it is a fundamental requirement to correctly estimate the duration of the tasks involved. While it is not difficult to assign the duration to the tasks in a scheduling software like MS Project or Primavera, estimating the duration of each task requires in-depth practical knowledge and proper calculation.

There are several techniques, some theoretical, and some practical by which we can estimate the task duration. In this article, we would focus much on practical methods. Of course, we would be touching on the theoretical methods as well to provide you with some ideas.

1. Three-Point Technique (with Historical Data)

What formula can a project manger use to estimate task duration? Let’s see in this part.

This is a very well-known technique of calculating the duration of a task that most schedulers are familiar with. This is a part of management courses all over the world. But the funny part is, in real life very few schedulers apply this. We believe that, unless you know the theory well you cannot utilize your practical knowledge to the fullest. Hence we will explain this technique with an example here.

The Three Point Technique to estimate the duration of a task utilizes historical data. Let us consider projects: A, B, C, and D, where tasks 1,2, and 3 were executed. We must have the tracked schedule of all four projects to perform this task.

As Step 1, We have obtained the data for the above projects from the tracked schedule. Now we will export the data in a spreadsheet to have more clarity on this.

Now, as the Step 2, we will find out the following three types of times for the completion of each of the tasks:

1. Optimistic time (best case scenario, when everything goes the right way) – denoted by ‘O’
2. Pessimistic time (worst case scenario, when everything goes the bad way) – denoted by ‘P’
3. Most likely time (the usual scenario) – denoted by ‘M’

The formula to find out the task duration by the Three-point technique is:

Duration of task = (O + 4M + P)/6

This is also called the PERT Formula.

This is a simple formula a Project Manager can use to estimate task duration. Before we implement the formula, let’s first see how to find out the optimistic time, pessimistic time, and the most likely scenario from historical data.

From the above historical data, we observe that the above three points are as below for tasks 1, 2, and 3:

To calculate the above, we have taken the least duration of the task as the optimistic time and the highest duration of the task as the pessimistic time. To calculate the ‘most likely’ time, we have made an average of the remaining two instances.

Now we will implement the above formula and estimate the duration of each task. The result is as per the table below:

If you need the spreadsheet, we have made it available through the link given below:

2. The Delphi Method: Consult the Subject Matter Experts and Get Benefit from their Experience

In case you don’t have the historical data to estimate the task duration of a project, there is an alternative!

Each construction company has its own bunch of experts in the field. So, you can simply circulate a spreadsheet with the task names on it to all such colleagues and get the most out of their experience. Let them opine on the possible duration of the tasks (with logic, of course) and you have a ready set of historical data at your disposal.

The steps of the Delphi Method is explained below for your better understanding:

Round-1: In order to acquire a wide knowledge of the experts’ perspectives on future events, ask generic questions. The questions might be distributed in the form of a questionnaire or survey. Collect and summarise the replies, deleting any extraneous information and looking for common points of view.

Round-2: The next round of questions should go deeper into the topic to clarify particular concerns based on the responses to the initial questions. These questions might potentially be sent as a questionnaire or survey. Collect and summarise the results once more, deleting any extraneous material and looking for common ground. Remember, we’re trying to reach an agreement.

Round-3: The final questionnaire is intended to aid in decision making. Concentrate on the areas of agreement. What is it that all the experts agree on?

You may prefer to have more than three rounds of inquiry to achieve a more certain conclusion. Once you get the data prepared, you can easily apply the three-point technique and prepare the first-cut schedule. Then keep a discussion session with the same with your most prominent colleagues in the field, and see wonders happen.

The only job left to you now is to screen the same keeping the contract duration intact. Adjust the available float that way.

Read More: What is Resource leveling in Project Management?

Practical Examples of Estimating Task Duration

Enough of theory, and let’s now discuss some real-life methods to estimate the task duration. We will use a few practical examples to do it, for the sake of easy understanding.

1. The Truss Lifting Task: Resource constraint method

I have a project where we need to lift structural steel pieces for the roof truss. We have enough space in the project to install only 3 tower cranes. It is not possible to lift the heavy steel members by any other means. In this case, the duration of the task is:

Duration of the task = Total number of lifts required / ( working hour per day / cycle time of 1 lift x no. of tower cranes in the project)

One point to note is, you need to add a little extra time to add a buffer. If you have time, refer to the historical data for the lift cycle time of a crane of similar capacity. Then apply the three-point technique to obtain what to consider in the above table.

2. Estimating the Duration of a Slab Formwork Removal Task

There is a specific technical rule for the formwork removal of Columns, Walls, Beams, and Slabs. The scheduler must follow the rule while estimating the duration of the task. For example, if the span of your slab is more than 4.5 m, you cannot remove the prop from the soffit of the slab before 14 days. Hence, you cannot start the block or brick masonry work within that period and it will govern the period of the task in this case.

Similarly, technically you need to cure the blockwork or brickwork for minimum 7 days, so you should plan for the plaster works keeping this in mind.

3. Considering Space Constraint While Estimating Duration of a Task

While estimating the task duration, pay attention to the limitation of maximum resource engagement. For example, you cannot engage much workers in a confined place. So, even if there is no resource constraints in your project, you cannot engage above a limit due to limitation of space.

The Best Principle

The estimation of the duration of a task can be done most appropriately only when there is a proper mix of statistical data and practical experience. Projects are unique in nature, so relying solely on the theoretical data is not a good choice. Also, when you are preparing the schedule for a time-bound project, keep in mind that you must maintain the contractual duration. You need to mobillize and optimize your resources that way.

Do you have any further points that we can consider to estimate the task durations of a project? Please share your idea in the comment box.