6.4 Estimate Activity Duration Process

The Estimate Activity Duration process results in an expected duration of each project activity.

This process is performed by the project manager, the project team, and often subject matter experts.

This process will occur at least once on every project, but will likely reoccur several times and may also occur concurrently with other processes, such as activity resource estimating.

Several of the outputs from other processes are needed for us to establish how long each activity is going to take.

These are the activity list and activity attributes, resource requirements, resource calendars, and the risks.

Estimate Activity Durations process

Estimate Activity Duration Process Decomposition

Estimate Activity Duration Process: Inputs

  • Activity list
    The activity list is the complete list of project activities that are needed to produce the work packages.
    It's decomposed from the WBS work packages. It’s the source document this process needs to identify what resources are required.
  • Activity attributes
    The activity attributes document is a companion to the activity list. It provides sufficient detail to fully describe the activity, and any supplementary information about activity, such as its relationships, constraints, assumptions, dependencies, and responsible people.
  • Activity resource requirements
    The activity resource requirements document describes the resource needs at the activity level, which can be aggregated up to the work package level. It focuses on the resource types and quantities needed.
  • Resource calendars
    This may be one or more calendars that identify when people, equipment, and material are available and for what lengths of time.
    For example, a resource calendar would indicate when supplies were expected to arrive and in what quantity. There is also a composite resource calendar that shows the availability of named human resources on the project as well as their skills.
    These calendars are useful for duration estimating because scarcity or unavailability of resources may increase the duration of activities.
  • Project scope statement
    Assumptions and constraints are factors in activity duration. The project scope statement details the measurable goals, objectives, deliverables, and requirements of the project, and what the acceptance criteria of deliverables will be.
    It also describes the work required to meet all objectives and deliverables of the project, and it also contains milestones, assumptions, risks, and costs.
  • Enterprise environmental factors
    Any of the many enterprise environmental factors and systems that influence the project should be considered when estimating activity duration.
    These factors can include its personnel, its organizational culture, its tolerance of risk, and its formal and informal hierarchy.
    Organizations may also have custom or commercial analytical databases that can include cost estimating, risk, or demographic data.
  • Organizational process assets
    Organizational process assets are the source of existing policies, processes, organizational data, and knowledge.
    These assets include the entire collection of formal and informal methodologies, policies, procedures, plans, and guidelines, as well as the organization's "knowledge base," which includes historical performance data, labor information, service and maintenance history, issue and defect history, project files, and financial data.

Estimate Activity Duration Process: Tools and Techniques

  • Expert judgment
    Expert judgment is based on the experience and knowledge of subject matter experts. It's used to assess and evaluate the inputs and the information they contain.
  • Analogous estimating
    Analogous estimating is a form of expert judgment that uses similar activities from past projects to provide duration or cost estimates.
  • Parametric estimating
    Parametric estimating uses mathematical formulas, usually involving quantity and productivity rates, to determine estimates.
  • Three-point estimates
    Three-point estimates provide a weighted average that helps level out some of the uncertainty in estimates. Three-point estimates use the optimistic, pessimistic, and most likely estimates.
  • Reserve analysis
    Reserves are time or cost buffers in the project schedule or budget that help the project respond to uncertainties. Reserve analysis monitors these buffers and will use, reduce, or eliminate them based on the current situation.

Estimate Activity Duration Process: Outputs

  • Activity duration estimates
    Activity duration estimates are the work periods required to complete a scheduled activity. There are many factors that influence duration, including resource availability, multi-tasking, and risks.
  • Project document updates
    Estimating activity duration may result in updates to project documents, including constraints, assumptions, and activity attributes.

Effort and Duration

The effort is the amount of labor required for an activity while duration is how long the activity will take expressed in a work period.

The effort needed for activities was estimated through the activity resource estimating process and is shown in the activity resource requirements document.

Duration is not just the effort converted into a different working time period. Though directly linked to effort, the duration is usually longer because of non-project time that is part of most people’s working day.

Since activity duration are the basis for the schedule, inaccuracies will result in an unrealistic schedule, so below are some generalizations affecting duration estimates that will help us establish better duration:

  • A number of resources​
    Activities with a large number of human resources will take somewhat longer because of the "overhead" involved in coordination and communication.
  • Work periods​
    The normal or acceptable work periods may be different between companies, offices, regions, personnel classifications or professions. Regulations may also be applicable which limit the work periods for some resource types.
  • Multiple shifts​
    Productivity levels are usually different between shifts for the same resource type.
  • Resource allocation​
    Most resources will not be able to devote their entire working day to project work. This is true in any organizational structure, but more of a factor in functional and matrix organizations.
  • Resource types
    Though experienced resources may be more efficient than those with less experience, resource availability or cost factors will influence what resource types can be used on which activities.
    Sometimes it’s necessary to opt for less-experienced personnel because of availability or scarcity, meaning it’ll usually take them longer to perform the same tasks.
  • Risk factors
    Risky activities need to be undertaken with more attention to detail, which likely means longer efforts and duration.
  • Multi-tasking
    Multi-tasking is a productivity loser, but it can’t always be avoided.
    Any resource who is juggling other job duties or performing multiple project activities simultaneously is multitasking.
    When we cannot rearrange the person’s tasks to eliminate multitasking, we need to allow longer duration for those resource's activities.
  • Funding limits
    Calendar-based funding limits, either established by the customer or performing organization, may restrict when work can be performed and can affect duration.
    For example, if the budget limit for November is $25,000, but there is $30,000 of scheduled activities planned, $5,000 of those scheduled activities is going to have to be adjusted, either by rescheduling the start date or limiting the number of resources performing those activities, which will increase duration.

Parkinson’s Law

Another contributor to inaccurate duration estimates is a situation known as Parkinson's Law. Parkinson's Law states that work expands to fill the time available.

This means that if we estimated an activity will take two weeks, it'll end up taking two weeks even if it could have been finished it in one. This isn't due to deliberate misinformation on anyone’s part.

For example, our two-week estimate might have included padding because we know some other things are also due during that two weeks, or we might have allowed extra time for risks that we knew might be involved in the activity.

We might also have just wanted to be absolutely sure we would meet that deadline, so we gave ourselves several extra days.

But the problem that arises from Parkinson's Law is that even when this "padding" is done with the best of intentions, it results in additional time that is invisible and unaccountable.

As project managers, we have no real idea whether the activity indeed took two weeks or only seven days, and this not only makes the activity duration on our project inaccurate but could lead to inaccuracies in other projects which rely on our project for historical data.

Estimating Techniques

There are three estimating approaches that can help make duration estimates more accurate.

These are analogous estimating, parametric estimating, and three-point estimates.

Activity Duration Estimating Techniques

Estimating Techniques

1. Analogous estimating is a form of expert judgment that relies on the duration of a similar past activity to predict how long another activity will take.

Care must be taken when using historical information from other projects because an activity can sometimes appear similar in project documentation but have actually been much different or have been performed under many different circumstances.

For example, the experience level of the other project team may have been lower or there could have been hidden padding in the activity's duration.

Still, analogous estimating is more accurate than estimates from the project team or expert judgment alone.

2. Parametric estimating uses mathematical formulas as the source for duration estimates.​

It isn’t applicable to all activities, but it can be used when the quantity of work and productivity rate are known. 

For example, if brochures can be produced at the rate of 1000 per hour, it will take 10 hours to print 10,000 brochures. As long as other activity factors and attributes are also taken into consideration, parametric estimates can be very accurate.

We have to account for uncertainty in our estimates, and so it’s common to use pessimistic, optimistic, and most likely values.

Pessimistic is the worst-case scenario, optimistic is the best-case scenario, and the most likely estimate is the one based on a reasonable situation. Standard deviation and three-point estimates can be used when the estimates that are coming from team members for the same activity are varying widely.

Standard deviation identifies how diverse the data population is. A high standard deviation indicates a lot of uncertainty in the estimates.

The formula for standard deviation used by the PMBOK is:

(Pessimistic Estimate - Optimistic Estimate) / 6

For example, the most pessimistic estimate received from the project team is 12 weeks, and the most optimistic estimate is 4 weeks.

The standard deviation for this activity is 1.33. (12 - 4) / 6 = 1.33

Some of the uncertainty in estimates can be factored out by using three-point estimates.

3.three-point estimate sometimes referred to as PERT, uses the three estimates and applies a mathematical formula.

The result is a duration estimate that is a weighted average.

The formula for a three-point estimate is:

(Pessimistic Estimate + (4 × Most Likely Estimate) + Optimistic Estimate) / 6

If the pessimistic estimate for an activity is 12 weeks, the optimistic estimate is 4 weeks, and the most likely is 7 weeks, then the three-point estimate is 7.33.

Reserve Analysis

Reserve Analysis

Reserve Analysis

Another scheduling tool that can be used is reserve analysis.

Reserve analysis looks at various factors in the project, such as risks and overall uncertainty, and sets aside separate contingency reserves, time reserves, or buffers that can be drawn from if activities exceed their duration.

Buffers can also achieve more accurate estimates by moving the padding out of the activity and into a separate bucket where it can be better managed.

Any buffers and the reasoning behind those buffers need to be fully documented. Whether this reserve time is added to the schedule as a whole, to individual activities, or two crucial points within the schedule depends upon the project manager and his or her scheduling approach.

But wherever they appear in the schedule, the best reserve amounts are ones that give just the right amount of breathing room because reserves need constant monitoring as unexpected items may draw down the contingency reserve too much or an excessive reserve may never be needed.

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