Modeling Tough Scheduling Problems with Project Management Software

Alex S. Brown, PMP IPMA-C

This paper was written to prepare for a large-group workshop on scheduling. Read the paper below, or jump straight to the exercises.

Despite year-over-year growth in the use and capabilities of Project Management software, many common scheduling problems remain difficult to represent and manage using these tools. Software alone cannot address fundamentally tough problems, including:

  • resource leveling
  • representing task dependencies — hard and soft
  • managing difficult-to-predict or quickly-changing work assignments
  • making schedules easy to maintain during project execution

Solving these problems require analysis of a project’s objectives, constraints, risks, and opportunities. They require a thinking human being to guide scheduling software.

Novice users of scheduling software have a difficult time with these types of problems. Their use of the software is driven by an urgent need to represent their schedule in a report, by any means possible. Advanced users of scheduling software, though, use the software to develop a model of their schedule. Eventually they will produce the same reports as a novice user, but their approach to the software is different.

Although techniques will vary with each scheduling software tool, some basic approaches are fundamental to building an accurate schedule. By running key scenarios through his or her software tool, a manager can help make progress from a novice to an expert user. Even expert users profit from these exercises, as a way to challenge old habits and try new techniques.

Novice Use of Scheduling Software

Novice users of scheduling software have a primary goal of communication. They have an urgent need to produce reports:

  • Gantt charts
  • Task lists
  • Team member reports
  • Executive reports

Their primary goal is to get the correct dates, duration, work, resources, and other project data into the tool and onto reports. Modern project management software allows the project manager to type desired values into a spreadsheet-like view to achieve this result quickly. The software often makes choices about dependencies, resource usage, and other factors, so reports appear on-screen quickly.

Communication is the critical factor in successful project management. Many managers never go beyond this “novice” phase; many never see the need. Even after years of experience as a project manager, many people’s scheduling skills remain at this level. Symptoms include (Uyttewaal 2001, 24-26):

  • Failure to maintain an up-to-date schedule during project execution; creating a schedule at the project start and never updating it afterwards
  • “Delicate schedules” whose end-dates change dramatically each time actual work is applied to them
  • Large amounts of time and frustration spent for each schedule change (new tasks, changed assignments, etc.)

Expert Use of Scheduling Software

Project managers with more expertise use scheduling software very differently. They create a model of the project in their software tool. They follow consistent steps to build up a schedule. They understand and override the default settings of their chosen software tool, manually specifying task dependencies and resource usage. Often they ignore predicted end-dates for tasks while entering key data. They wait until their schedule is complete and resource-leveled, then optimize the schedule to reach a desired end-date.

Expert schedulers run “what-if” scenarios on their schedules. They modify start and end dates for critical tasks; they ensure that the remaining project work moves in a realistic manner. They understand what their software will do if key tasks are actually done out-of-order. They regularly update their schedule based upon actual work completed, predicting a new end-date for the overall effort when necessary.

Every project manager comes up with his or her own habits when working with scheduling software. Scheduling software and experience drive these habits. Managers must develop a new mental map of the software. Experienced users view their software as a database manager, not a spreadsheet. They see their project files as a database of related facts, with many views to see the data from different angles. (Stover 2003, 7-8) They consult many views, including the network diagram and resource usage reports, even if they are never published.

Establishing Key Terminology

To allow users of different scheduling software tools to talk to each other, it is necessary to go back to basic definitions. The PMBOK provides definitions for standard terms. Different software packages use different terminology, but in some way they all represent network diagrams, task lists, Gantt charts, finish-to-start dependencies, and so on. This paper uses definitions from PMBOK.

To discuss common scheduling problems, this paper defines two key classes of projects:

  • Parallel opportunities
  • Serial dependencies

Real projects often contain a mix of these situations. Some projects follow one pattern for some phases and the other pattern for other phases. Scheduling these two types of projects, though, demand different approaches.

  • A network diagram with parallel opportunities. At the left is a Start milestone linked to eleven tasks, stacked top to bottom to the right. Only one task, number three, has a dependency; number twelve must follow it. Number twelve is to the right of three. All the tasks except three link to a Finish milestone to the far right.Parallel Opportunities

The network diagram for a parallel-opportunities project contains many tasks that could be done at the same time. Software enhancement projects often follow this pattern; each software component could be designed or constructed independently. At some point all changes need to be integrated, but each change can be worked independently for a large part of the project. The number of available resources typically drives the end-date for these projects.

The network diagram for a serial-dependency project requires that tasks be done in a specific order. Construction projects often follow this pattern, where walls cannot be erected until the foundation is poured and set, or where walls cannot be completed until wiring, pipe, and key inspections are complete. The total duration of the critical path typically drives the end-date for these projects.

  • A network diagram with serial dependencies. A very wide network diagram of twelve tasks, showing tasks one to seven linked in a row, and eight to twelve linked in a row below. Task twelve is dependent on task three as well.Serial Dependencies
  • Workflow for Modeling a Schedule

    By breaking schedule development into well-defined, discrete processes, the PMBOK suggests a workflow for building a schedule. In real life, building a schedule is often an iterative process, starting with a small, basic schedule, and building on it by adding phases or groups of tasks. Whether performed as a single pass or multiple iterations, experienced project managers perform these steps:

  1. Build a Work Breakdown Structure (WBS)
  2. Decompose the WBS into concrete activities
  3. Define the sequence of activities (dependencies, predecessors, and successors)
  4. Estimate task duration
  5. Estimate task cost
  6. Assign resources
  7. Adjust cost and duration estimates based upon assigned resources
  8. Perform initial resource leveling
  9. Optimize the schedule

The final step, “optimize”, may include changes to all of the information entered earlier. Activities might be grouped together, activities might be decomposed, resource assignments may change, and estimates of cost or duration may grow or shrink.

During this “optimization” step, the manager probes the schedule, assessing the quality of the model of the schedule. Changing assignments and estimates should adjust the overall end-date with minimal effort. The better the model, the easier it is to optimize. The more fragile the schedule, the more unpredictable changes occur with each adjustment.

Often modern schedules are tightly constrained. They are designed to produce the maximum output, just-in-time, for a minimum budget. Optimizing tightly constrained schedules is difficult. Allow extra time for creating such schedules.

In the rush to produce a schedule, it is tempting to bypass critical steps. For instance, if a task “must” begin on a certain date, it is tempting to ignore activity sequencing for any predecessor to that task and to simply set a start date for it. If the manager shortcuts the process, the software cannot warn the manager of scheduling conflicts.

Some Tough Scheduling Problems

Resource Leveling

One of the enormous benefits of scheduling software is that it provides the manager with multiple, synchronized views of the project. Gantt charts are effective for communicating dates and network diagrams are effective for communicating dependencies. In the 1960s and 1970s, managers who used critical-path techniques had to manually draw and update these diagrams by hand. Any changes to the project scope meant updates to multiple diagrams, charts and lists. Today’s software automates this essential function. Users may edit data in one view, and the software will display up-to-date versions of all other views at the click of a button.

Theoretically, these advances should make it easy to keep a resource-leveled schedule. Indeed, modern software provides views that show the utilization of any resource in just about any unit, cumulative by date or for a particular period. With the entry of vacation calendars, holidays, resource availability over time, and other constraints, software can often show overallocation or underallocation of available resources automatically.

The key to accurate resource leveling is accurate, complete data. So many variables come together in this critical planning task:

  • Resource start dates and availability
  • Work estimates
  • Duration estimates
  • Activity sequence relationships
  • Vacation schedules
  • Holiday schedules
  • Planning factors for unscheduled but known events like sick days
  • Planning for schedule risk events

Resource leveling is the integration point for all these project elements.

Complexity increases with the use of scheduling software. It is possible to force a schedule to be resource-leveled strictly through the use of dependency relationships in most tools. Some tools offer automated systems to adjust resource allocations. Project managers should explore their tools thoroughly to understand the best techniques for their projects. Key questions for the project manager include:

  • Is the method repeatable for future projects and for future reporting periods with this project?
  • What if a key resource starts earlier or later than expected? How much data must be adjusted or re-entered?
  • What if a single task takes more or less time than expected? How will the software respond?
  • When there are two tasks that COULD start on the same date (but resource constraints will not allow them to), how does the software decide which one SHOULD start first? Does the software make decisions automatically? Can the project manager override any automated decisions easily?

Scheduling software is often designed for projects with serial dependencies. By adding “soft” dependencies to a schedule, it is possible to turn a project with many parallel opportunities into one with mostly serial dependencies. If a manager uses this technique,

  • Where will the hard dependencies be documented?
  • Will the software still accurately calculate key project measures, including critical path?
  • When resources do work out-of-order, does the manager need to update the schedule? How will the software respond if the actuals do not match the planned activity sequence?

The better the manager understands his or her software tool, the less time is required for schedule maintenance.

Representing Task Dependencies — Hard and Soft

Task sequencing is often challenging with scheduling software. Some software will add sequencing rules to a schedule, making all tasks sequential automatically; many expert schedulers disable these features.

PMBOK distinguishes between dependencies that MUST be enforced (hard or mandatory) and dependencies that SHOULD ideally be enforced (soft or discretionary) (PMBOK 2000, 68-69). Many software packages do not make that distinction, enforcing all dependencies strictly. Some resource-leveling strategies require the use of many soft dependencies, compounding the problem.

Managers have a few options to work around these common problems:

  • Use the software to enforce ONLY hard dependencies, and attempt to meet soft dependencies as part of resource-leveling steps
  • Create an initial network diagram with only hard dependencies and maintain it outside the main schedule (especially useful when the project’s network diagram changes infrequently)
  • Ignore the software’s network diagram and create one by hand
  • Keep a spreadsheet or diagram of hard dependencies and soft dependencies – compare the schedule in the software to these lists regularly and update the software as needed
  • Create custom views or use custom fields in the software to track hard and soft dependencies

Task numbering is often an obstacle in creating and synchronizing lists of tasks and dependencies. Most software numbers all tasks sequentially, and adjusts the number of every task as tasks are added or deleted. Often there is a hidden field that does not change that uniquely identifies each task. Use this number to effectively cross-reference between a spreadsheet or diagram and the data in the scheduling software. Alternatively, create task names that are unique within the project, and use these names as an effective cross-reference.

Managing and Maintaining Changing Schedules

“Everything flows on and on like this river, without pause, day and night.” -Confucius, standing by a river (Wilhelm 1987, lv)

The one unchanging principle that every project manager can count on is change itself. No matter how accurate the estimates, no matter how perfect the schedule, before the project is done, the schedule will change. Some tasks will start or end early, some will start or end late, and the project scope will expand and contract. Managing this change is the never-ending role of the project manager.

Managing schedule change is a requirement for many tough schedules. Fast-changing resource assignments, frequent changes to project scope, changes to budget, and changes to desired end-dates can make a project much more difficult to manage.

When creating a schedule for a brand new project, a manager should ask:

  • What is likely to change?
  • How much will it change?
  • Will project changes come all at once, or a little with every reporting period?

The more the manager can anticipate change, the better he or she can plan for it. When creating the initial schedule, the manager can perform some “what-if” scenarios, seeing the effect of certain types of changes will be. Even if the exact changes cannot be predicted, the manager can develop scheduling techniques to address common sources of change:

  • New tasks
  • Old tasks no longer needed
  • Resources are unavailable or new resources become available
  • Resource start dates changing
  • Changes to duration, work, and cost estimates

An expert scheduler can draw upon experience to address these situations. A novice needs to develop these techniques or learn from an expert. No matter what the experience level, though, any manager can benefit from identifying likely sources of changes and planning for them.

Modern software has opened new possible ways to model change, making uncertainty an explicit part of the project schedule. Software has been able to generate likely ranges of estimates based upon PERT estimates of likely, optimistic, and pessimistic outcomes for years. Monte Carlo simulation can be used to show the range of possible outcomes, and their relative probability. Many project managers do not need or use these tools, but for highly uncertain projects, they can be an essential feature of scheduling software.


Project Management Institute. (2000) A Guide To The Project Management Body Of Knowledge (PMBOK®) (2000 ed.). Newtown Square, PA: Project Management Institute.

Stover, Teresa (2003). Microsoft Project 2002 Inside Out. Redmond, WA: Microsoft Press.

Uyttewaal, Eric (2001). Dynamic Scheduling with Microsoft Project 2000. Ottawa, ON: International Institute for Learning, Inc.

Wilhelm, Hellmut (1987). The I Ching or Book of Changes: The Richard Wilhelm Translation (3rd ed.) (C.F. Baynes, Trans. To English). Princeton, NJ: Princeton Univ. Press (Original translation from Chinese to German by Richard Wilhelm in 1923).