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Estimating Process and Methods
Quick Summary
Provides an overview of project estimating methods.
What this is
This guideline provides an overview of estimating methods for generating detailed project schedules. It includes a brief description of the purpose, goals, and process of estimating, as well as specific estimating techniques for the project manager and project team.
Methods of estimating covered in this guideline are:
- Historical Data
- Decomposition Process
- Brainstorming meetings
- Heuristics (Personal or "Expert" Experience)
- Delphi Method
- Network Diagrams - PERT, CPM (ADM), GERT, etc.
- Rule of Thumb registries and charts
- "Rolling wave" or "iterative" estimating
Why it's useful
Schedules are composed of inter-related tasks, milestones and deliverables. Tasks are assigned to various project team members to accomplish. Because the project schedule has interdependencies between tasks, and ultimately a critical path (the longest time-path of interdependent tasks through a project), estimates of various tasks affect the end-date and we'd like to have as good an estimate of those tasks as possible.
Many times, the person doing the task may not be able to provide a good estimate. Having various methods or alternatives to generating an estimate can help the project team come up with the best possible estimate
Desired or required levels of confidence in the estimates provided by the PM and project sometimes changes during the progress of a project. Early in the project planning, the desired goal may be 70% confidence for routine tasks and 80% for critical tasks. This may change to 85% and 90% as the project completes the planning phase and nears the end of the design phase. Note: these numbers are just examples. Your company will have its own confidence levels. Also, remember that a "level of confidence" is, in many cases, only an opinion, possibly tainted by the opinion bearer's optimism or knowledge base.
As the project progresses from the concept phase into more intense and detailed planning, super-tasks will be broken down into detailed, sub-tasks and these different levels of detail may need different estimation methods.
How to use it
Review the estimating process overview.
Review the tools suggested and choose one or more of them for your project estimation process. Some of the tools are complex, like the PERT Network Diagramming method. These network diagram methods and their use is outside the scope of this document. They are identified here for the project manager to decide if further separate investigation of those techniques is necessary.
Some of the tools are simple. Some of them are reminders of things you may already know or may have forgotten. None of these is a silver bullet that will magically produce excellent estimates. Using various methods, and comparing the estimates achieved using different methods, will improve your project plan estimates.
Remember that cost, work effort and time are three elements of your simple task estimates. Risk estimation is not specifically called out in this document, but estimates for the effects of risk avoidance or risk actualization can be, in some cases, estimated using some of these methods.
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