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Activity Duration Estimating


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 resource calendars.

Overall project duration is calculated as an output of the Schedule Development process (Section 6.5).

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6-8. : Activity Duration Estimating: Inputs, Tools & Techniques, and Outputs

Section 6.4.1 Activity Duration Estimating: Inputs

.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One or more of the organizations involved in the project may maintain duration estimating databases and other historical reference data.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 4.1.1.4) 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 4.1.1.4) 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 (Section 5.2.3.1) 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 6.1.3.1.

.5 Activity Attributes

Described in Section 6.1.3.2.

.6 Activity Resource Requirements

The estimated activity resource requirements (Section 6.3.3.1) 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 resources (Section 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 through

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 information (Section 4.1.1.4) 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 11.4.2.2.). 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 assumptions.


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