6.3.1 Inputs to Activity Duration Estimating

.1 Activity list. The activity list is described in Section 6.1.3.1.

.2 Constraints. Constraints are described in Section 6.1.1.4.

.3 Assumptions. Assumptions are described in Section 4.1.1.5. An example would be reporting periods for duration of the project that could dictate maximum durations, i.e., two reporting periods.

.4 Resource requirements. Resource requirements are described in Section 7.1.3.1. The duration of most activities will be significantly influenced by the resources assigned to them. 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 communication overload, wich reduces productivity and causes production to improve proportionally less than the increase in resource.

.5 Resource capabilities. The duration of most activities will be significantly influenced by the capabilities of the humans and material resources assigned to them. For example, if both 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.

.6 Historical information. Historical information 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 duration estimating databases—historical 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 does it take concrete to cure; how long does a government agency usually take to respond to certain types of requests).

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

.7 Identified risks. The project team considers information on identified risks (see Section 11.2) when producing estimates of activity durations, since risks (either threats or opportunites) can have a significant influence on duration. The project team considers the extent to which the effect of risks is included in the baseline duration estimate for each activity, including risks with high probabilities or impact.

6.3.2 Tools and Techniques for Activity Duration Estimating

.1 Expert judgment. Expert judgment is described in Section 5.1.2.2. Durations are often difficult to estimate because of the number of factors that can influence them (e.g., resource levels, resource productivity). Expert judgment guided by historical information should be used whenever possible. If such expertise is not available, the estimates are inherently uncertain and risky (see Chapter 11, Project Risk Management).

.2 Analogous estimating. Analogous estimating, also called top-down 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 (described in Section 6.3.2.1).
  Analogous estimating is most reliable when (a) the previous activities are similar in fact and not just in appearance, and (b) the individuals preparing the estimates have the needed expertise.

.3 Quantitatively based durations. The quantities to be performed for each specific work category (i.e., number of drawing, meters of cable, tons of steel, etc.) defined by the engineering/design effort, when multiplied by the productivity unit rate (i.e., hours per drawing, meters of cable per hour, etc.), can be used to estimate activity durations.

.4 Reserve time (contingency). Project teams may choose to incorporate an additional time frame, called time reserve, contingency, or buffer, that can be added to the activity duration or elsewhere in the schedule as recognition of schedule risk. This reserve time can be a percentage of the estimated duration, or a fixed number of work periods. The reserve time 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.

6.3.3 Outputs from Activity Duration Estimating

.1 Activity duration estimates. 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 8 days and no more than 12 (assuming a five-day workweek).

   15 percent probability of exceeding 3 weeks to indicate a high probability—85 percent—that the activity will take 3 weeks or less.

  Chapter 11 on Project Risk Management includes a more detailed discussion of estimating uncertainty.

.2 Basis of estimates. Assumptions made in developing the estimates must be documented.

.3 Activity list updates. Activity list updates are described in Section 6.2.3.2.