Project Schedule Estimating

Project Schedule Estimating entails developing the work activity schedule and associated activity relationships and dependencies that will be needed for the organization to conduct the project or otherwise provide the product, service, or system deliverables. This practice presents the fundamental concepts and technique guidance for estimating the project schedule.

Conceptually ...

This practice enables the project manager and project planners to assign estimated values for work effort duration to project work elements. In turn, the duration (i.e.,& the project schedule) can be tracked and controlled against those estimates.

Project Schedule Estimating is normally conducted during project planning (Plan Phase). However, schedule estimates should be continuously reviewed and updated as necessary throughout the project. Please refer to the key events or indicators (listed on the Project Estimating practice area introduction page) that should automatically trigger follow-on reviews (and possible updates) of project schedule estimates.

Schedule Development Overview--To a large extent, "schedule development" and "schedule estimating" mean the same thing and are interchangeable terms.

However, we should recognize that some authorities would use the term "schedule estimating" to indicate a focus on the determination of work activity duration. In this methodology, "schedule estimating" refers to all aspects of schedule development.

The project schedule guides the expected duration of the project work effort, helps identify the number and types of required resources, and establishes a baseline for project tracking and controlling. The primary objective of project schedule development is to ensure that the project is completed on time. Project schedule development is linked to the processes for developing the project scope, WBS, cost estimates, and resource utilization estimates. Changes in any of these areas are likely to affect the schedule.

Scheduling terminology is used to give planning team members a common frame of reference for developing the project schedule. The following scheduling terms are frequently used:

Activity--A work effort performed in a specified period of time that corresponds to a work element (normally a work package) contained in the project WBS.

Activity Duration Estimate--A quantitative assessment of the duration of work periods or the amount of time needed to complete an activity.

Activity Relationship--The relationship that exists between one or more project activities, as illustrated for the six numbered activities:

Finish-to-Start-Activity 1 must be totally finished before activity 2 can start. This is the most common activity relationship, with lag or lead time added as needed.

Start-to-Start-Activity 3 cannot start until activity 2 begins, or they can actually begin at the same time.

If needed, a lag or lead period can be specified.

Finish-to-Finish-Activity 4 cannot finish until activity 3 finishes, or they can actually finish at the same time. If needed, a lag or lead period can be specified.

Start-to-Finish-Activity 5 must start before activity 6 can finish. This is a much less common relationship than the other three. Lag or lead time adds more complexity.

Dependency--logical relationship among activities in a project's WBS; an external dependency is a logical relationship to work activities of another project or program.

Lag Time --An adjustment to a logical relationship in the project schedule such that there is a delay in the start time of a subsequent activity.

Lead Time--An adjustment to a logical relationship in the project schedule such that there is acceleration in the start time of a subsequent activity.

Slack Time (or Float)--The amount of time during which no activity is performed, as determined by the logical relationships of work activities; this is also the difference between when an activity can start (its earliest start time) and when it must start (its latest start time before causing a project delay).

Float greater than zero means that time is available along a particular activity path.

Float equal to zero means the activities along that path are critical.

Float less than zero means that the project is behind schedule, critically late, and re-planning may be necessary.

Milestone --An activity having duration of zero time.

Activity relationships. 1 2 3 4 5 6

Determine Activity Sequencing

Activity sequencing identifies and documents dependencies among project activities. It represents not only activity dependencies but also task constraints. Activities must be sequenced accurately to support development of a realistic and achievable schedule.

The project planning team performs activity sequencing to determine the predecessor and successor activities as well as to specify the types of relationships (start-to-finish, start to-start, finish-to-start, and finish-to-finish, along with any required lag or lead time). The following are general steps for accomplishing activity sequencing:

The project planning team members review the work elements of the WBS (i.e., activities, tasks, subtasks, etc.) to examine the required flow of work activity.

The team then establishes the activity relationship dependencies for each work element at the lowest, work package level, as a means to achieve the desired work activity flow.

Display activity sequencing by creating a network diagram, either manually or through use of an available project management software application package.

In association with activity sequencing, the project planning team may find it necessary to make adjustments to the elements of the WBS, which is done by adding, deleting, or modifying project work elements in the WBS. The following are common methods for displaying activity sequencing:

Precedence Diagramming Method (PDM)--This method of constructing a project network diagram uses nodes to represent the activities and arrows connecting the nodes to show the dependencies. This is sometimes called the activity-on-node (AON) diagram, and this method is found in many project management soft ware packages.

Arrow Diagramming Method (ADM)--This method of constructing a project network diagram uses arrows to represent the activities and connections at nodes to show the dependencies. This is sometimes called the activity-on-arrow (AOA) diagram. It’s a technique that is less common than PDM but may still be used in some application areas. ADM uses only finish-to start dependencies and may require the use of "dummy activities" (e.g., "start" and "finish" activities) to properly present all logical relationships.

Conditional Diagramming Method (CDM)--This method of constructing a project network diagram enables non-sequential activities such as loops or conditional branches to be created. A prime example of this technique is the Graphical Evaluation and Review Technique (GERT).

Project Activity Templates --Standard activity tem plates can be used to expedite the preparation of project network diagrams. These templates can include an entire project or just a component of it. They are useful when projects include some recurring identical or nearly identical work efforts that are repeated on each project.

Estimate Activity Duration Activity duration estimating involves determining the working time that will be needed to complete each specified project activity. If the work effort is resource driven, then the duration will depend on the number of people assigned to do the work. Therefore, it’s critical that this process be con ducted concurrently with resource utilization estimating. In some cases, the duration of a task can be shortened by adding resources to it, but not in all cases.

Estimating duration usually considers elapsed time, which represents the calendar days, weeks, or months that pass from the project start date to the project finish date. Estimating duration also considers working time, where weekends and holidays are considered, as well as vacation, other non-work days, and general availability of the resources needed for the work effort.

Estimating activity durations requires that each project activity be examined to determine how much time is needed to complete it. Planners must discern between elapsed time and working time, and sometimes must resolve issues created by each type of duration estimate:

If you are estimating based on elapsed time, then the work activity inherently requires the passage of time to enable an activity to start or be completed. Some examples of using elapsed-time estimating considerations could include:

Awaiting shipment or delivery of required items on a specified date

Awaiting resource availability (e.g., vacation, hiring, not yet assigned)

Awaiting paint to dry (it requires some minimum period of elapsed time)

Awaiting action or document review and approval responses over a specified period of time (or over the period of time the review process actually takes)

Awaiting product development over a specified period of time (or over the period of time the development process actually takes)

Awaiting a specified activity or process (of known duration) to be fulfilled

It can be seen that items estimated using elapsed time generally require their block of time for completion to be specified. This is not a situation where you can complete the activity sooner by using additional resources.

Elapsed time activities will take just as long whether there are 3 or 10 resources available or assigned to the effort.

If you are estimating based on working time, then work estimates are considered relative to the level of effort. Some examples of using working time estimating considerations could include:

Loading and unloading materials on the delivery truck

Moving furniture; painting the walls

Developing lines of software code

Conducting product or system testing

System, subsystem, and equipment installations

Writing a technical or business document

It can be seen that items estimated using working time generally represent the level of effort of one or more individuals and that they are presumably qualified to perform the assigned project activity. To that end, working time activities can be completed sooner by increasing the level of effort of the individual or by increasing the number of resources assigned to accomplish the particular project activity.

Some common methods used for estimating activity durations include:

Historical Information --Durations that resulted when performing similar work efforts in the past can be reviewed and applied to activities in the current project work effort.

Expert Judgment --Durations are often difficult to estimate because of the number of factors that can influence them ( For example, resource levels and resource productivity). Expert judgment (guided by historical information) can be used to sort out the complexities and determine duration estimates. Such expertise may be provided by any group or individual with specialized knowledge or training in the type of work that is planned. This expertise is available from a variety of sources, including other units within the performing organization, consultants, professional and technical associations, and industry groups. Sometimes this expertise is conveyed by means of written materials rather than by individuals.

Analogous Estimating --This method is often referred to as "top-down estimating." Analogous estimating uses the estimated duration of a larger, broader scope activity as the basis for estimating the durations of any of its component activities. This method can also be used when higher-level activity duration is "mandated," and durations of one or more subordinate activities must contribute to achieving that "mandate." Simulation --Simulation involves calculating multiple durations with different sets of assumptions. A common simulation method is Monte Carlo analysis, in which a distribution of probable results is defined for each activity and used to calculate a distribution of probable results for the total project.

Develop the Project Schedule and Critical Path

The project schedule is developed when each project work activity has a specified duration estimate for completion, the necessary relationship alignment with predecessor and successor activities has been created, and the project critical path has been identified.

The Critical Path is defined as the longest logical path through the project network diagram. It’s the path with zero float (slack), and also the path that provides the shortest time to complete the project.

Most project management scheduling software will calculate and display the critical path for a project. The following steps can be used to manually calculate the critical path:

Establish a project "Start" activity and align all successive project activities relative to their predecessors and successors (i.e., create and examine the network diagram).

Establish a project "End" activity (i.e., the final activity).

Conduct a forward pass through the network to determine the early start time for each activity. The early start is the earliest an activity can start based on the defined logic.

Determine the natural end date based on the results of the forward pass through the network.

Conduct a backward pass through the network to determine the late start for each activity. The late start is the latest an activity can start without delaying the project completion date.

Calculate float (slack).

Identify the critical path along the path from project start to project finish that has zero float (slack); also examine any other paths having low levels of float.

Use project network management techniques to eliminate any negative float.

Insert hard constraints to override normal critical path calculations and remove naturally occurring float.

Hard constraints can also be used to create project milestones and position project activities to eliminate excessive float.

It should be noted that calculations could result in a project having more than one critical path. This is not unusual for large, complex projects. However, a final examination should be performed to determine if the schedule can be reduced to one single critical path.

Make Subsequent Schedule Adjustments

There are a few actions that can be taken to improve project schedule efficiency, as follows:

"Crashing" the Project Network --Crashing means accelerating the completion of tasks on the critical path to complete the project earlier or, in the event of a delay, to resume the original schedule. This is an intensive, iterative process having the following steps:

Determine the desired total project duration.

Examine and prioritize the project activities along the critical path.

Specify the crash (duration reduction) time desired or needed for each high-priority project activity along the critical path.

Select a high-priority activity and determine ways in which a time reduction can be achieved.

Apply changes to reduce the selected activity's duration; then examine how much the total project duration has been brought closer to the desired time.

Verify the critical path as remaining the same examine consequences if the critical path is changed; and, if changed, restart the "crashing" effort at step 2 in this list.

Continue reduction of the selected priority activity until the desired activity "crash" time is attained.

Proceed to the next priority activity and repeat this process to continue "crashing" (duration reduction) until the desired total project duration is achieved.

Optimizing Resource Utilization--Activity duration can be reduced in the following ways:

• Assigning resources to work overtime to complete the activity sooner.



• Using more resources to increase the level of work effort.



• Increasing productivity by using improved technology, or improving the process involved in accomplishing the work effort Increasing productivity by using resources with higher skill and qualifications.

"Fast Tracking" --The project schedule can be adjusted (reduced) by performing some of the project activities in parallel (fast tracking) instead of one after the other (in sequence). The schedule can also be adjusted by changing the relationship among activities until the critical path is shorter.

Establish the Project Schedule Baseline

Upon completion of project schedule (duration) estimates, the results are usually forwarded to the project executive (sponsor) and perhaps other management entities (e.g., PMO) for review and approval. Otherwise, the project manager may have the authority to approve the project activity durations (project schedule) as per standard procedures within the organization or as specified in the Project Charter.

In some situations, the customer may also be a reviewer of early schedule estimates.

A schedule baseline is established when the review authority has approved the schedule estimates. The schedule baseline is normally set by the project manager, and any changes to this baseline must go through a change management process. The schedule baseline is normally established in conjunction with setting the cost and resource utilization baselines.

When using project management software, there is normally an automated feature or function available to establish and save the approved schedule estimates as a part of the project baseline.