Activity Duration Estimating
The process of estimating schedule activity durations uses
information on schedule activity scope of work, required resource types,
estimated resource quantities, and resource calendars with resource
availabilities. The inputs for the estimates of schedule activity duration
originate from the person or group on the project team who is most familiar with
the nature of the work content in the specific schedule activity. The duration
estimate is progressively elaborated, and the process considers the quality and
availability of the input data. For example, as the project engineering and
design work evolves, more detailed and precise data is available, and the
accuracy of the duration estimates improves. Thus, the duration estimate can be
assumed to be progressively more accurate and of better quality.
The Activity Duration Estimating process requires that the amount
of work effort required to complete the schedule activity is estimated, the
assumed amount of resources to be applied to complete the schedule activity is
estimated, and the number of work periods needed to complete the schedule
activity is determined. All data and assumptions that support duration
estimating are documented for each activity duration estimate.
Estimating the number of work periods required to complete a
schedule activity can require consideration of elapsed time as a requirement
related to a specific type of work. Most project management software for
scheduling will handle this situation by using a project calendar and
alternative work-period resource calendars that are usually identified by the
resources that require specific work periods. The schedule activities will be
worked according to the project calendar, and the schedule activities to which
the resources are assigned will also be worked according to the appropriate
Overall project duration is calculated as an output of the
Schedule Development process (Section 6.5).
6-8. : Activity Duration Estimating: Inputs,
Tools & Techniques, and Outputs
Section 6.4.1 Activity Duration
.1 Enterprise Environmental Factors
One or more of the organizations involved in the project may
maintain duration estimating databases and other historical reference data. This
type of reference information is also available commercially. These databases
tend to be especially useful when activity durations are not driven by the
actual work content (e.g., how long it takes concrete to cure or how long a
government agency usually takes to respond to certain types of requests).
.2 Organizational Process Assets
Historical information (Section 18.104.22.168) on the likely
durations of many categories of activities is often available. One or more of
the organizations involved in the project may maintain records of previous
project results that are detailed enough to aid in developing duration
estimates. In some application areas, individual team members may maintain such
records. The organizational process assets (Section 22.214.171.124) of the performing
organization may have some asset items that can be used in Activity Duration
Estimating, such as the project calendar (a calendar of working days or shifts
on which schedule activities are worked, and nonworking days on which schedule
activities are idle).
.3 Project Scope Statement
The constraints and assumptions from the project scope statement
126.96.36.199) are considered when estimating the schedule activity durations. An
example of an assumption would be the length of the reporting periods for the
project that could dictate maximum schedule activity durations. An example of a
constraint would be document submittals, reviews, and similar non-deliverable
schedule activities that often have frequency and durations specified by
contract or within the performing organization’s policies.
.4 Activity List
Described in Section 188.8.131.52.
.5 Activity Attributes
Described in Section 184.108.40.206.
.6 Activity Resource Requirements
The estimated activity resource requirements (Section 220.127.116.11) will have an effect
on the duration of the schedule activity, since the resources assigned to the
schedule activity, and the availability of those resources, will significantly
influence the duration of most activities. For example, if a schedule activity
requires two engineers working together to efficiently complete a design
activity, but only one person is applied to the work, the schedule activity will
generally take at least twice as much time to complete. However, as additional
resources are added or lower skilled resources are applied to some schedule
activities, projects can experience a reduction in efficiency. This
inefficiency, in turn, could result in a work production increase of less than
the equivalent percentage increase in resources applied.
.7 Resource Calendar
The composite resource calendar (Section 6.3), developed as part of
the Activity Resource Estimating process, includes the availability,
capabilities, and skills of human resources (Section 9.2). The type, quantity,
availability, and capability, when applicable, of both equipment and materiel
12.4) that could significantly influence the duration of schedule activities
are also considered. For example, if a senior and junior staff member are
assigned full time, a senior staff member can generally be expected to complete
a given schedule activity in less time than a junior staff member.
.8 Project Management Plan
The project management plan contains the risk register (Sections 11.2
11.6) and project cost estimates (Section 7.1).
Risk Register. The risk register has
information on identified project risks that the project team considers when
producing estimates of activity durations and adjusting those durations for
risks. The project team considers the extent to which the effects of risks are
included in the baseline duration estimate for each schedule activity, in
particular those risks with ratings of high probability or high impact.
Activity Cost Estimates. The project
activity cost estimates, if already completed, can be developed in sufficient
detail to provide estimated resource quantities for each schedule activity in
the project activity list.
Section 6.4.2 Activity Duration
Estimating: Tools and Techniques
.1 Expert Judgment
Activity durations are often difficult to estimate because of the
number of factors that can influence them, such as resource levels or resource
productivity. Expert judgment, guided by historical information, can be used
whenever possible. The individual project team members may also provide duration
estimate information or recommended maximum activity durations from prior
similar projects. If such expertise is not available, the duration estimates are
more uncertain and risky.
.2 Analogous Estimating
Analogous duration estimating means using the actual duration of a
previous, similar schedule activity as the basis for estimating the duration of
a future schedule activity. It is frequently used to estimate project duration
when there is a limited amount of detailed information about the project for
example, in the early phases of a project. Analogous estimating uses historical
18.104.22.168) and expert judgment.
Analogous duration estimating is most reliable when the previous
activities are similar in fact and not just in appearance, and the project team
members preparing the estimates have the needed expertise.
.3 Parametric Estimating
Estimating the basis for activity durations can be quantitatively
determined by multiplying the quantity of work to be performed by the
productivity rate. For example, productivity rates can be estimated on a design
project by the number of drawings times labor hours per drawing, or a cable
installation in meters of cable times labor hours per meter. The total resource
quantities are multiplied by the labor hours per work period or the production
capability per work period, and divided by the number of those resources being
applied to determine activity duration in work periods.
.4 Three-Point Estimates
The accuracy of the activity duration estimate can be improved by
considering the amount of risk in the original estimate. Three-point estimates
are based on determining three types of estimates:
Most likely. The duration of the schedule
activity, given the resources likely to be assigned, their productivity,
realistic expectations of availability for the schedule activity, dependencies
on other participants, and interruptions.
Optimistic. The activity duration is based
on a best-case scenario of what is described in the most likely estimate.
Pessimistic. The activity duration is
based on a worst-case scenario of what is described in the most likely estimate.
An activity duration estimate can be constructed by using an
average of the three estimated durations. That average will often provide a more
accurate activity duration estimate than the single point, most-likely estimate.
.5 Reserve Analysis
Project teams can choose to incorporate additional time
referred to as contingency reserves, time reserves or buffers, into the overall
project schedule as recognition of schedule risk. The contingency reserve can be
a percentage of the estimated activity duration, a fixed number of work periods,
or developed by quantitative schedule risk analysis (Section 22.214.171.124.). The contingency
reserve can be used completely or partially, or can later be reduced or
eliminated, as more precise information about the project becomes available.
Such contingency reserve is documented along with other related data and
Section 6.4.3 Activity Duration
.1 Activity Duration Estimates
Activity duration estimates are quantitative assessments of the
likely number of work periods that will be required to complete a schedule
activity. Activity duration estimates include some indication of the range of
possible results. For example:
2 weeks ± 2 days to indicate that the schedule activity will
take at least eight days and no more than twelve (assuming a five-day workweek).
15 percent probability of exceeding three weeks to indicate
a high probability—85 percent—that the schedule activity will take three weeks
.2 Activity Attributes (Updates)
The activity attributes (Section 126.96.36.199) are updated to
include the durations for each schedule activity, the assumptions made in
developing the activity duration estimates, and any contingency reserves.