39 Estimating Techniques

Resources are people, equipment, place, money, or anything else that you need in order to do all of the activities that you planned for. Every activity in your activity list needs to have resources assigned to it. Before you can assign resources to your project, you need to know their availability. Resource availability includes information about what resources you can use on your project, when they’re available to you, and the conditions of their availability. Don’t forget that some resources, like consultants or training rooms, have to be scheduled in advance, and they might only be available at certain times. You’ll need to know this before you can finish planning your project. If you are starting to plan in January, a June wedding is harder to plan than one in December, because the wedding halls are all booked up in advance. That is clearly a resource constraint. You’ll also need the activity list that you created earlier, and you’ll need to know how your organization typically handles resources. Once you’ve got a handle on these things, you’re set for resource estimation.

Estimating the Resources

The goal of activity resource estimating is to assign resources to each activity in the activity list. There are five tools and techniques for estimating activity resources.

Expert judgment means bringing in experts who have done this sort of work before and getting their opinions on what resources are needed.

Alternative analysis means considering several different options for how you assign resources. This includes varying the number of resources as well as the kind of resources you use. Many times, there’s more than one way to accomplish an activity and alternative analysis helps decide among the possibilities.

Published estimating data is something that project managers in a lot of industries use to help them figure out how many resources they need. They rely on articles, books, journals, and periodicals that collect, analyze, and publish data from other people’s projects.

Project management software such as Microsoft Project will often have features designed to help project managers estimate resource needs and constraints and find the best combination of assignments for the project.

Bottom-up estimating means breaking down complex activities into pieces and working out the resource assignments for each piece. It is a process of estimating individual activity resource need or cost and then adding these up together to come up with a total estimate. Bottom-up estimating is a very accurate means of estimating, provided the estimates at the schedule activity level are accurate. However, it takes a considerable amount of time to perform bottom-up estimating because every activity must be assessed and estimated accurately to be included in the bottom-up calculation. The smaller and more detailed the activity, the greater the accuracy and cost of this technique.

Estimating Activity Durations

Once you’re done with activity resource estimating, you’ve got everything you need to figure out how long each activity will take. That’s done in a process called activity duration estimating. This is where you look at each activity in the activity list, consider its scope and resources, and estimate how long it will take to perform.

Estimating the duration of an activity means starting with the information you have about that activity and the resources that are assigned to it, and then working with the project team to come up with an estimate. Most of the time you’ll start with a rough estimate and then refine it to make it more accurate. You’ll use these five tools and techniques to create the most accurate estimates:

Expert judgment will come from your project team members who are familiar with the work that has to be done. If you don’t get their opinion, there’s a huge risk that your estimates will be wrong.

Analogous estimating is when you look at similar activities from previous projects and how long they took. This only works if the activities and resources are similar.

Parametric estimating means plugging data about your project into a formula, spreadsheet, database, or computer program that comes up with an estimate. The software or formula that you use for parametric estimating is based on a database of actual durations from past projects.

Reserve analysis means adding extra time to the schedule (called a contingency reserve or a buffer) to account for extra risk.

Three-point estimating or PERT  is when you come up with three numbers: a realistic estimate that’s most likely to occur, an optimistic one that represents the best-case scenario, and a pessimistic one that represents the worst-case scenario. The final estimate is the weighted average of the three.

How PERT calculator works?

      • PERT Formula:
      • TE = (O + 4T + P) / 6
      • where,
      • TE = Pert Expected Time Duration,
      • O = Optimistic estimate,
      • P = Pessimistic estimate,
      • T = Typical estimate
      • PERT Standard Deviation (SD) Formula:
      • SD = (P – O) / 6
      • where,
      • SD = Standard Deviation,
      • P = Pessimistic estimate,
      • O = Optimistic estimate


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Project Management Basics Copyright © by Sharon Blanchard is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.