How do you estimate project task duration?

When you create project plans, you need to assess the time required to complete individual activities. Use duration estimates to get an idea of this time and determine the schedule you need to adhere to in order to complete projects. You can use various methods to estimate activity duration, depending on the nature of the activities.

PERT Method

The Program Evaluation and Review Technique, or PERT, estimation method is a weighted average of three scenarios. First, compute the averages for optimistic, pessimistic and most likely time scenarios. PERT gives more weight to the most likely scenario, so multiply that average by four. Combine this result with the optimistic and pessimistic averages, and divide the result by six to come up with a final estimate. The optimistic scenario presumes all goes well, and you can complete the activity without issues. Conversely, with the pessimistic scenario, you imagine everything will go wrong and it’ll take longer to complete the activity. The most likely scenario assumes you can complete the activity without surprises. The PERT estimation method is a good option if you’re uncertain about the activities. It also enables you to consider all associated risks.

Analogous Estimation

The analogous, or top-down, estimation relies on information from similar projects to determine the activity duration for a current project. You’ll need historical data and a degree of expertise about the similar projects, because the reliability of your estimation depends on how closely the activities match the projects you're using as comparisons. Use this method at the beginning of a project when you don’t have all the details. Adjust the estimates as you learn more about the tasks and how long they can be expected to take with the resources available.

Parametric Estimation

The parametric estimation is similar, but more accurate, than the analogous estimation. To use it, multiply the number of units you need by the time it takes to produce the units. You’ll need historical information about similar activities to complete your estimate. The method is scalable. This means if your historical data tells you that it takes one person an hour to produce one unit, you can reasonably estimate that you can complete three units within one hour if you allocate three workers to the task. When you use this method, it's important to account for all tasks that impact the activity. For example, if the workers spend part of the time preparing materials, account for that time in your estimates.

Expert Judgment

If your project is complicated and a number of factors can influence the duration of your activities, you might want to use expert judgment to estimate activity duration. Experts knowledgeable in a particular area can judge the time and resources you’ll need to complete activities in that area. Be careful when choosing experts or using their estimates, because they may have certain biases that might influence their estimates. You also can gather estimates from external experts. If you can get external estimates at reasonable cost, it might be prudent to rely on them instead of generating estimates internally.

The Delphi Technique

The Delphi technique makes use of group intelligence to determine activity duration. The technique involves gathering opinions from several experts and then sending the responses back to the experts for their review. They can change their opinions after reviewing the responses. The process might involve several rounds, as you want to investigate differences of opinion and get to a consensus. To reduce bias and prevent individual experts from overly influencing results, experts submit their opinion anonymously. You can use a third party to gather the opinions.

Work Breakdown Structure

Certain activities might be too large or complex for a reliable duration estimate. If an activity takes up more than 10 percent of the project schedule, you might want to break it into several different tasks. You can use a work breakdown structure to reduce these activities into smaller, more manageable tasks. Doing this enables you to set priorities and estimate the duration of tasks more accurately. A work breakdown structure also is useful for building accountability, because you can assign specific tasks to designated project participants.

References

Image Credit

Patrick Ryan/Digital Vision/Getty Images

10 MIN READ

Calculating Realistic Project Timelines

© GettyImages
Clu

Run projects like clockwork.

Have you ever been on a project where the deadline was way too tight?

Chances are that tempers were frayed, sponsors were unhappy, and team members were working ridiculous hours. Chances are, too, that this happened because someone underestimated the amount of work needed to complete the project.

People often underestimate the amount of time needed to implement projects, particularly when they're not familiar with the work that needs to be done.

For instance, they may not take into account unexpected events or urgent, high-priority work; and they may fail to allow for the full complexity of the job. Clearly, this will likely have serious negative consequences further down the line.

This is why it's important to estimate time accurately if your project is to be successful. In this article, we explore some of the methods that you can use for making good time estimates.

Why Estimate Time Accurately?

Accurate time estimation is a crucial skill in project management. Without it, you won't know how long your project will take, and you won't be able to get commitment from the people who need to sign it off.

Even more importantly for your career, sponsors often judge whether a project has succeeded or failed depending on whether it has been delivered on time and on budget. To have a chance of being successful as a project manager, you need to be able to negotiate sensible budgets and achievable deadlines.

How to Estimate Time Accurately

Use these steps to make accurate time estimates:

Step 1: Understand What's Required

Start by identifying all of the work that needs to be done within the project. Use tools such as Business Requirements Analysis, Work Breakdown Structures, Gap Analysis, and Drill-Down to do this in sufficient detail.

As part of this, make sure that you allow time for meetings, reporting, communications, testing, and other activities that are critical to the project's success. (You can find out more about these activities in our article, Project Management Phases and Processes.)

Step 2: Order These Activities

Now, list all of the activities you identified in the order in which they need to happen.

At this stage, you don't need to add in how long you think activities are going to take. However, you might want to note any important deadlines. For example, you might need to get work by the finance department finished before it starts work on year-end.

Subscribe to Our Newsletter

Receive new career skills every week, plus get our latest offers and a free downloadable Personal Development Plan workbook.

Read our Privacy Policy

Step 3: Decide Who You Need to Involve

You can do the estimates yourself, brainstorm them as a group, or ask others to contribute.

Where you can, get the help of the people who will actually do the work, as they're likely to have relevant experience to draw upon. By involving them, they'll also take on greater ownership of the time estimates they come up with, and they'll work harder to meet them.

Tip:

It's always a good idea to consult with others about your project. You can confirm your assumptions with them, and it can encourage initiative. Doing so will also ensure that you don't miss something crucial – for fear of looking weak or feeling threatened by team members. See our article, Avoiding Management Self-Sabotage, for more on this.

Step 4: Make Your Estimates

You're now ready to make your estimates. We've outlined a variety of methods below to help you do this. Whichever methods you choose, bear these basic rules in mind:

• To begin with, estimate the time needed for each task rather than for the project as a whole.
• The level of detail you need to go into depends on the circumstances. For example, you may only need a rough outline of time estimates for future project phases, but you'll probably need detailed estimates for the phase ahead.
• List all of the assumptions, exclusions and constraints that are relevant, and note any data sources that you rely on. This will help you when your estimates are questioned, and will also allow you to identify any risk areas if circumstances change.
• Assume that your resources will only be productive for 80 percent of the time. Build in time for unexpected events such as sickness, supply problems, equipment failure, accidents and emergencies, problem solving, and meetings.
• If some people are only working part-time on your project, bear in mind that they may lose time as they switch between their various roles.
• Remember that people are often overly optimistic, and may significantly underestimate the amount of time that it will take for them to complete tasks.

Note:

It's likely that your estimate following these principles will be longer than you want or are permitted to allow. But don't be tempted to be unrealistic and knowingly underestimate.

Instead, look at removing wasteful steps using the Lean and 5S models. You may also be able to increase capacity, so that work can be done in parallel, or choose automated processes over manual ones.

See our sections on Strategy, Team Management and Time Management for further guidance on supporting everyone in their individual productivity.

Methods for Estimating Time

We'll now look at different approaches that you can use to estimate time. You'll probably find it most useful to use a mixture of these techniques:

• Bottom-Up Estimating

  Bottom-up estimating allows you to create an estimate for the project as a whole. To analyze from the "bottom up," break down larger tasks into detailed tasks, then estimate the time needed to complete each one.

  Because you're considering each task individually, your estimate of the time required for each one is likely to be more accurate. You can then add up the total amount of time needed to complete the plan.

  Tip 1:

  How much detail you go into depends on the situation. However, the more detailed your analysis is, the more accurate it will be.

  If you don't know how far to go, consider breaking work into chunks that one person can complete in half a day, for example. This may seem like a rather "circular" approach, but it will give you an idea of the level of detail you should aim for.

  Tip 2:

  Yes, this does take a lot of work; however, this work will pay off later in the project. Just make sure that you leave plenty of time for it in the project's Design Phase.

• Top-Down Estimating

  In top-down analysis, you develop an overview of the expected timeline first, using past projects or previous experience as a guide.

  It's often helpful to compare top-down estimates against your bottom-up estimates, to ensure accuracy.

  Note:

  Don't assume that the bottom-up estimates are wrong if they differ widely from the top-down ones. In fact, it's more likely that the reverse is true.

  Instead, use the top-down estimates to challenge the validity of the bottom-up estimates, and to refine them as appropriate.

• Comparative Estimating

  With comparative estimating, you look at the time it took to do similar tasks, on other projects.

• Parametric Estimating

  With this method, you estimate the time required for one deliverable, then multiply it by the number of deliverables required.

  For example, if you needed to create pages for a website, you'd estimate how much time it would take to do one page, then multiply this time by the total number of pages to be produced.

• Three-Point Estimating

  To build in a cushion for uncertainty, you can do three estimates – one for the best case, another for the worst case, and a final one for the most likely case.

  Although this approach requires additional effort to create three separate estimates, it allows you to set more reasonable expectations, based on a more realistic estimate of outcomes.

• Planning Poker in Agile Project Management

  Estimating anything inevitably involves guessing. Planning Poker is a tool to help you guess better. It's a more rough-and-ready and less time-consuming approach, to help plan short sprints of work in Agile PM.

Tip:

In the early stages of project planning, you often won't know who will do each task – even though this can influence how long the task will take. For example, an experienced developer should be able to develop a software module much more quickly than someone less experienced.

You can build this into your estimates by giving best, worst, and most likely estimates, stating the basis for each view.

Apply Your Estimates

Once you've estimated the time needed for each task, you can prepare your project schedule. Add your estimates to the draft activity list that you produced in the second step, above.

You can then create a Gantt Chart to schedule activities and assign resources to your project, and to finalize milestones and deadlines.

Tip:

If your project is complex, it's crucial that you highlight the tasks that can't be delayed if you're to hit your deadline. Use Critical Path Analysis and PERT Charts to help you do this.

Key Points

You need to estimate time accurately if you're going to deliver your project on time and on budget. Without this skill, you won't know how long your project will take, and you won't be able to get commitment from the people required to help you achieve your objective.

More than this, you risk agreeing to impossibly short deadlines, with all of the stress, pain, and loss of credibility associated with this.

To estimate time effectively, follow this four-step process:

1. Understand what's required.
2. Prioritize activities and tasks.
3. Decide who you need to involve.
4. Do your estimates.

Use a variety of methods to get the most accurate time estimates.