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Schedule Development: Tools and Techniques


Schedule Development: Tools and Techniques

.1 Schedule Network Analysis

Schedule network analysis is a technique that generates the project schedule. It employs a schedule model and various analytical techniques, such as critical path method, critical chain method, what-if analysis, and resource leveling to calculate the early and late start and finish dates, and scheduled start and finish dates for the uncompleted portions of project schedule activities. If the schedule network diagram used in the model has any network loops or network open ends, then those loops and open ends are adjusted before one of the analytical techniques is applied. Some network paths may have points of path convergence or path divergence that can be identified and used in schedule compression analysis or other analyses.

.2 Critical Path Method

The critical path method is a schedule network analysis technique that is performed using the schedule model. The critical path method calculates the theoretical early start and finish dates, and late start and finish dates, for all schedule activities without regard for any resource limitations, by performing a forward pass analysis and a backward pass analysis through the project schedule network paths. The resulting early and late start and finish dates are not necessarily the project schedule; rather, they indicate the time periods within which the schedule activity should be scheduled, given activity durations, logical relationships, leads, lags, and other known constraints.

Calculated early start and finish dates, and late start and finish dates, may or may not be the same on any network path since total float, which provides schedule flexibility, may be positive, negative, or zero. On any network path, the schedule flexibility is measured by the positive difference between early and late dates, and is termed “total float.” Critical paths have either a zero or negative total float, and schedule activities on a critical path are called “critical activities.” Adjustments to activity durations, logical relationships, leads and lags, or other schedule constraints may be necessary to produce network paths with a zero or positive total float. Once the total float for a network path is zero or positive, then the free float — the amount of time that a schedule activity can be delayed without delaying the early start date of any immediate successor activity within the network path — can also be determined.

.3 Schedule Compression

Schedule compression shortens the project schedule without changing the project scope, to meet schedule constraints, imposed dates, or other schedule objectives. Schedule compression techniques include:

.4 What-If Scenario Analysis

This is an analysis of the question “What if the situation represented by scenario ‘X’ happens?” A schedule network analysis is performed using the schedule model to compute the different scenarios, such as delaying a major component delivery, extending specific engineering durations, or introducing external factors, such as a strike or a change in the permitting process. The outcome of the what-if scenario analysis can be used to assess the feasibility of the project schedule under adverse conditions, and in preparing contingency and response plans to overcome or mitigate the impact of unexpected situations. Simulation involves calculating multiple project durations with different sets of activity assumptions. The most common technique is Monte Carlo Analysis (Section 11.4.2.2), in which a distribution of possible activity durations is defined for each schedule activity and used to calculate a distribution of possible outcomes for the total project.

.5 Resource Leveling

Resource leveling is a schedule network analysis technique applied to a schedule model that has already been analyzed by the critical path method. Resource leveling is used to address schedule activities that need to be performed to meet specified delivery dates, to address the situation where shared or critical required resources are only available at certain times or are only available in limited quantities, or to keep selected resource usage at a constant level during specific time periods of the project work. This resource usage leveling approach can cause the original critical path to change.

The critical path method calculation (Section 6.5.2.2) produces a preliminary early start schedule and late start schedule that can require more resources during certain time periods than are available, or can require changes in resource levels that are not manageable. Allocating scarce resources to critical path activities first can be used to develop a project schedule that reflects such constraints. Resource leveling often results in a projected duration for the project that is longer than the preliminary project schedule. This technique is sometimes called the resourcebased method, especially when implemented using schedule optimization project management software. Resource reallocation from non-critical to critical activities is a common way to bring the project back on track, or as close as possible, to its originally intended overall duration. Utilization of extended hours, weekends, or multiple shifts for selected resources can also be considered using different resource calendars to reduce the durations of critical activities. Resource productivity increases are another way to shorten durations that have extended the preliminary project schedule. Different technologies or machinery, such as reuse of computer code, automatic welding, electric pipe cutters, and automated processes, can all have an impact on resource productivity. Some projects can have a finite and critical project resource. In this case, the resource is scheduled in reverse from the project ending date, which is known as reverse resource allocation scheduling, and may not result in an optimal project schedule. The resource leveling technique produces a resource-limited schedule, sometimes called a resource-constrained schedule, with scheduled start dates and scheduled finish dates.

.6 Critical Chain Method

Critical chain is another schedule network analysis technique that modifies the project schedule to account for limited resources. Critical chain combines deterministic and probabilistic approaches. Initially, the project schedule network diagram is built using non-conservative estimates for activity durations within the schedule model, with required dependencies and defined constraints as inputs. The critical path is then calculated. After the critical path is identified, resource availability is entered and the resource-limited schedule result is determined. The resulting schedule often has an altered critical path.

The critical chain method adds duration buffers that are non-work schedule activities to maintain focus on the planned activity durations. Once the buffer schedule activities are determined, the planned activities are scheduled to their latest possible planned start and finish dates. Consequently, in lieu of managing the total float of network paths, the critical chain method focuses on managing the buffer activity durations and the resources applied to planned schedule activities.

.7 Project Management Software

Project management scheduling software is widely used to assist with schedule development. Other software might be capable of interacting directly or indirectly with project management software to carry out the requirements of other Knowledge Areas, such as cost estimating by time period (Section 7.1.2.5) and schedule simulation in quantitative risk analysis (Section 11.4.2.2). These products automate the calculation of the mathematical forward pass and backward pass critical path analysis and resource leveling, and, thus, allow for rapid consideration of many schedule alternatives. They are also widely used to print or display the outputs of developed schedules.

.8 Applying Calendars

Project calendars (Section 4.1.1.4) and resource calendars (Section 6.3.3.4) identify periods when work is allowed. Project calendars affect all activities. For example, it may not be possible to work on the site during certain periods of the year because of weather. Resource calendars affect a specific resource or category of resources. Resource calendars reflect how some resources work only during normal business hours, while others work three full shifts, or a project team member might be unavailable, such as on vacation or in a training program, or a labor contract can limit certain workers to certain days of the week.

.9 Adjusting Leads and Lags

Since the improper use of leads or lags can distort the project schedule, the leads or lags are adjusted during schedule network analysis to develop a viable project schedule.

.10 Schedule Model

Schedule data and information are compiled into the schedule model for the project. The schedule model tool and the supporting schedule model data are used in conjunction with manual methods or project management software to perform schedule network analysis to generate the project schedule.


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(Posted on April 20, 2011, 11:06 AM sabelo hlubi)