ACTIVITY DURATION ESTIMATING

The process of estimating activity durations uses information on activity scope, required resource types, and estimated resource quantities. The inputs for the estimates of 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 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 known quality.

Activity duration estimating requires that: a) the amount of effort required to complete the activity be estimated, b) the assumed amount of resources to be applied to complete the activity be estimated, and c) the number of work periods needed to

complete the activity be determined. All data and assumptions that support the estimating of durations need to be documented for each duration estimate.

Estimating the number of work periods required to complete an activity can require consideration of elapsed time as a requirement related to a specific type of work. Most project management scheduling software will handle this situation by using alternative work-period calendars that are usually identified by the resources that require specific work periods. The activities to which the resources are assigned will then be worked according to the appropriate calendars.

Overall project duration can be estimated using the tools and techniques presented here, but it is more properly calculated as the output of schedule development (Section 6.5).

6.4.1 Activity Duration Estimating: Inputs

Described in Section 6.1.

.2 Activity List Attributes Described in Section 6.1.

.3 Project Scope Statement

The constraints and assumptions from the project scope statement (Section 5.2) need to be considered when estimating the activity durations.An example of an assumption to be considered would be reporting periods for the duration of the project that could dictate maximum activity durations, e.g., two reporting periods.

.4 Project Cost Estimates

Many project cost estimates (Section 7.1) are developed in sufficient detail to provide estimated resource quantities for each activity in the project activity list.

.5 Activity Resource Requirements

The estimated activity resource requirements (Section 6.3) will have an effect on the duration of the activity, since the resources assigned to the activity and the availability of those resources will significantly influence the duration of most activities. For example, two people working together may be able to complete a design activity in half the time it takes either of them individually, while a person working half time on an activity will generally take at least twice as much time as the same person working full time.

However, as additional resources are added, projects can experience a reduction in efficiency. This inefficiency in turn, could result in a production increase being less than the equivalent percentage increase in resources applied.

.6 Resource Availability

The availability, capabilities and skills of the human resources, and the type, capability, quantity and availability of equipment resources, and type, quantity and availability of materiel resources will significantly influence the duration of most activities. 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 activity in less time than a junior staff member (Section 9.1).

.7 Organizational Process Assets

Historical information (Section 4.1) on the likely durations of many categories of activities is often available from one or more of the following sources:

• Project Files. 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.

• Commercial Databases. Duration estimating databases and other historical reference data and information is often 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; how long a government agency usually takes to respond to certain types of requests).

• Project Team Knowledge. The individual members of the project team may remember previous results or estimates. While such recollections may be useful, they are generally far less reliable than documented results.

.8 Risk Register

The project team considers information on identified project risks (Section 11.2) when producing estimates of activity durations. The project team considers the extent to which the effects of risks are included in the baseline duration estimate for each activity, in particular those risks with ratings of high probability or high impact.

6.4.2 Activity Duration Estimating: Tools and Techniques

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. If such expertise is not

available the estimates are uncertain and risky (Chapter 11).

.2 Analogous Duration Estimating

Analogous duration estimating, also called top-down duration estimating, means using the actual duration of a previous, similar activity as the basis for estimating the duration

of a future activity. It is frequently used to estimate project duration when there is a limited amount of detailed information about the project (e.g., in the early phases).

Analogous estimating is a form of 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 Quantitatively Based Durations

Estimating the basis for activity durations can be quantitatively determined by multiplying the quantity of work performed by the productivity rate. For example:

Engineering/design effort: number of drawings times hours per drawing Cable installation: meters of cable times labor hours per meter

These total resource quantities are then 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. For example, total human resource labor hours times work hours per work period divided by number of that specific human resources that can be applied.

.4 Three-Point Estimates of Duration

The accuracy of the activity duration estimate can be improved by considering the risk in the original estimate. Three-point estimates are based on completing three types of estimates:

• Most likely. The duration of activity given the resources likely to be assigned, their productivity, realistic expectations of availability for the activity,

dependencies on other participants, and interruptions on the type of activities.

• Optimistic. The duration is based on a best-case scenario of what is described in the most likely estimate.

• Pessimistic. The duration is based on a worse case scenario of what is described in the most likely estimate.

An activity duration estimate can be constructed by using some average of the three estimated durations that average will often provide a more accurate estimate than the single point most likely estimate.

.5 Reserve Time

Project teams can choose to incorporate additional time referred to as time reserve, contingency, or buffer, into the overall schedule as recognition of schedule risk. Reserve time 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). The reserve time can be used completely or partially, or can later be reduced or eliminated, as more precise information about the project becomes available. Such reserve time should be documented along with other data and assumptions.

.6 Maximum Activity Duration

Experts recommend the maximum duration for any scheduled activity not exceed one reporting cycle. Exceptions might include submittals, reviews and similar non-deliverable activities that often have durations specified by contract or within the performing

organization’s policies.

6.4.3 Activity Duration Estimating: Outputs

Activity duration estimates are quantitative assessments of the likely number of work periods that will be required to complete an activity. Activity duration estimates should always include some indication of the range of possible results. For example:

• 2 weeks ± 2 days to indicate that the 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 activity will take three weeks or less.

.2 Activity List (Updates)

If activity lists are used, the activity duration estimating process can result in both addition and deletion of planned activities within the list.

.3 Activity List Attributes (Updates)

Assumptions made in developing the activity duration estimates are documented.