How do we come up with good schedule estimates?

How do we come up with good time estimates for all the different tasks in our project schedule?
Good estimating comes from experience, but you don't have to wait for your own experience to grow. Rely on those with greater experience—your team members or others in functional groups—to help with estimating task work effort and duration. You can also get input from other project managers who have worked on similar efforts. Another approach is to ask for best, worst, and most likely estimates for various tasks. (Document assumptions so everyone understands where those estimates came from!) You then produce a range of overall estimates for the entire project and discuss where to peg the schedule. See our guideline Estimating Process and Methods for more detail on different approaches, which include:
  • Industry rules of thumb
  • Low-Medium-High estimates
  • Top down vs. bottom up estimating and comparisons
  • Historical data from your company
  • Techniques for "sizing" various tasks and deliverables

Estimation should be done very visibly, with the active involvement of the people who will actually be executing the work. The person responsible for delivering an activity must commit to the work and cost estimates. These people know at the working level what it takes to get the job done, what their work environment is like, what tools and other supporting resources are needed, and how that will all affect the work time and costs to get the job done. Estimating work for each item in the project's work breakdown structure helps ensure the schedule develops methodically based on the hours or work truly required, rather than through top-down guesses about activity durations.

Here are some key pointers for the estimating process. See our guideline Planning and Scheduling: Estimating Work and Costs for more detail on each and guidance for all your schedule estimation work.

  • Ensure you've identified everyone who should be involved in the process.
  • Prepare the team for the estimating process.
  • Have each team member make a first pass at estimating work for each activity assigned to them.
  • Keep an eye on activity "sizes." The larger the piece of work, the more inaccurate the estimate may be.
  • Identify items whose effort estimates have a high degree of uncertainty and mark as schedule risks, so you can decide later to leave extra time.
  • Don't spend a lot of time on the first pass at estimating. Trade-offs will have to be made. A first pass helps you see how close or how far you could be from a plan that meets the desired end date.
  • Get commitments. The estimates don't mean much if the people doing the work aren't committed to the schedule.

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