7.3 Estimating Activity Durations

After we define the lowest level of activities in an activity list, each activity is reviewed and evaluated to determine the duration (how long it will take to accomplish from beginning to end) and what resources (e.g., human resources, materials, facilities, and equipment) are needed. An estimate is an educated guess based on knowledge, experience, and inference—the process of deriving conclusions based on assumptions. The estimate’s accuracy is related to the quality of the knowledge and how that knowledge is applied. The person with the most knowledge may not be the most objective person to provide duration estimates. The person responsible for the work may also want extra time. Therefore, multiple inputs into the duration estimate and a more detailed WBS help reduce bias—making decisions based on a prejudged perspective.

It is important here to highlight how a milestone differs from an activity. A milestone is a significant point or event in a project. Milestones have zero duration because they represent a significant point or event. A milestone list identifies all project milestones and indicates whether the milestone is mandatory, such as those required by contract, or optional, such as those based on historical information^[1].

The unit of time used to develop the activity duration is a function of the level of detail needed by the user of the schedule. The larger and more complex the project, the greater the need for detail, which usually translates into shorter durations for activities. However, it is common to use two types of units – one is days or weeks for activities, and hours to display the work hours.

In this book, we can elaborate on five types of estimation methods:

1. Expert judgment: The project team consults domain and implementation subject matter experts who have technical knowledge and experience in the areas the project activities are related to. If we are developing a new mobile application, we can consult software engineers, developers and testers, and systems analysts who were involved in activities to develop other mobile applications. They can provide us with information regarding scheduling estimates for each activity we are planning to carry out.

2. Appropriation method (Analogous estimating): Actual durations from similar projects are reviewed, and the same proportions are applied to the current project. However, internal and external factors that affected the previous projects, as well as those that may affect the current project, should be taken into account. Identified risks, with their probability and impact considered, could significantly influence duration estimates for the current project.

3. Parametric estimating: In this estimation technique, we can use equations and algorithms to calculate the duration based on the resources we use and how many hours they need to work or how many of them we need to use. This method is quantitative. We can multiply the quantity of work to be performed by the number of hours per unit of work. If we can estimate the amount of work, we can divide it by the amount of work that can be done in an hour. For example, let’s assume that, in our m-commerce project, we estimated that software developers need to create 200 lines of code for a module. Based on the previous projects and the feedback from the subject matter experts, we have estimated that a developer can finish 40 lines in an hour. Therefore, a developer needs 5 hours to finish all 200 lines. We can also add a one-hour break and two-hour review for this task. Therefore, the total work hours amount to 8, translating to one day in our schedule. Another example is the installation of cables in an infrastructure project. If workers can install 100 feet of cable per hour, the duration required to install 1,000 feet would be 10 hours (1,000 feet divided by 100 feet per hour). This technique can produce higher levels of accuracy depending on the sophistication and underlying data built into the model. Parametric schedule estimates can be applied to a total project or segments of a project in conjunction with other estimating methods^[2]. Entering data about the project into a formula, spreadsheet, or computer program produces a duration estimate by extrapolating information from a database of actual durations from past projects.

4. Three-point estimates: Duration estimates are done based on three scenarios:
   1. A realistic estimate (most likely to occur – m)
   2. An optimistic estimate (best-case scenario – o)
   3. A pessimistic estimate (worst-case scenario – p)

Two distributions are possible in the three-point estimation method – triangular and beta. In triangular distribution, all three duration estimates get the same weight. In a beta distribution, the realistic estimate gets four-sixths of the weight, whereas the other two estimates have one-sixth of the weight.

Triangular distribution:

Beta distribution:

Let’s estimate the duration of the “1.3 Preparation of Project Charter” sub-activities. Our team gathered together in a meeting to review and discuss alternative durations. We also consulted subject matter experts who work in relevant departments in our organization and external stakeholders with an interest and/or power. Our organization and the team had implemented software and website development projects and several mobile application development projects. Therefore, we have reports on the realized durations and lessons learned. So, we can start with analogous estimating first. As we have already worked on similar projects, that would facilitate the estimation process both for schedule and budget. We can place the historical information in the Most Likely column. Based on the expert judgment, lessons learned, and discussions during our team meeting, we determined pessimistic and optimistic durations in Table 7.3.1.

The computation of the duration for Activity 1.3.2 is shown below:

We can roll the duration estimates down or up to the nearest integral number.

5. WBS method (Bottom-up estimating): In this method, we start from the lowest level activities in the WBS – work packages. After we estimate the duration for all six activities (Table 7.3.1), we can find the duration of the parent activity, 1.3 “Preparation of Project Charter.” Adding duration for all six activities wouldn’t be an operation of adding all numbers (4+5+6+2+8+5), equal to 30 days. We must consider the dependencies between the activities, detailed in sections 7.4 and 7.5 of this chapter. Hence, we can find 22 days for 1.3. The same bottom-up estimating is applied to the activities (1.1., 1.2, 1.3, 1.4, 1.5), and the duration for “1. Scope” is 35 days. “1.6 Completion of the Scope Phase” is a milestone with zero duration. The overall duration for “1. Scope” is computed as 35 days, as seen in Table 7.3.2.