Problems, Methods and Tools of
Advanced Constrained Scheduling
Session 9
Shane Archibald
Archibald Associates & Spider Project (USA)
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Advanced Constrained Scheduling
• In this session, we will
– Review project constraints that must be taken into
account when a project schedule is created
– Review problems and challenges of constrained
scheduling
– Review results of “constrained” schedules from three
applications
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Critical Path Method
• Developed in the middle of the last century for
calculating project schedules
• Takes into account only network dependencies and
known estimates of activity durations
• *Assumes UNLIMITED resources*
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Critical Chain Method
• Introduced in the late ’90’s
• Recognizes that resources are limited and impact
completion of tasks
• Emphasizes the importance of the people completing
the work rather than the work itself; PEOPLE are the
critical elements of a project
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CPM “Work-Arounds”
• Use “Roles” and/or generic resources
• Pad activities (and costs)
• Preferential Logic
• Spend a lot of time explaining the above
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CPM “Work-Arounds”
• Manage schedule/resource changes “offline” or on an
ad-hoc basis
• Use other tools
• Avoid resource loading… cost loading… reporting from
the schedule…
• Spend a lot of time reintegrating information
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Realistic Project Schedules
• Project schedules must to take into account ALL
schedule constraints including:
– Renewable & consumable resource availability at any
given time
– Material and equipment availability at any given time
– Sufficient financing at any given time
– Schedule change must be feasible!
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What Information SHOULD be
Considered?
• Resource Availability by Time Period
• Resource & Crew Productivity Rates
• Material Requirements and Availability
– Including those consumed by resources
• Task Information
• Activity Dependencies
• Costs and Cost Components
• Project or Physical Conditions
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Renewable Resource Information That
Should be Considered…
• Renewable are those resources that can be used again
after they finish executing some activity
• Schedules should consider…
– Quantities available by period
– Calendars (regular time / overtime)
– Costs per regular and overtime work hour
– Skills (what types of work may be performed by these
resources)
– Other constraints
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Consumable Resource Information
That Should be Considered…
• Consumable resources are spent on project activities
(materials or installed equipment, for example)
• Schedules should consider…
– Costs per unit that may be different in different
periods
– Materials that are consumed by Resources or
Equipment
*Consumable Resources can also be produced (supplies)
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Consumable Resource Information
That Should be Considered…
• Consumable Resources can also be produced on
activities (supplies)
•
•
•
•
Financing / Cash Flow
Fuel
Concrete
Etc.
• If consumable resource supplies are limited, these
limitations become schedule constraints
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Cost Information That Should be
Considered…
• Cost Components
– Salaries
– Materials
– Machines
– Indirect Costs
• Cost Centers
– Expenses / Management / Contractors /
Subcontractors / Etc.
• Financing / Revenue
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Task Information That Should be
Considered…
• Resource & Crew Productivity Rates Should Determine
Activity Duration
Activity Duration = Volume/Total Productivity
Total Productivity = (R1N *R1 P)+ (R2N *R2 P)+… (RnN *Rn P )
– where Rx = Resource x, N = number of resources, P – productivity of
resources
(R1N = Number of Resource 1’s; R1 P = Productivity of Resource 1’s)
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Task Information That Should be
Considered…
• Interruptible vs. Non-Interruptible Activities
• ASAP / ALAP
• Costs
• Work / Activity Calendars
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Task Dependency Information That
Should be Considered…
• Standard Dependencies (SS, SF, FS, FF)
• Strict dependencies
– MUST start immediately following satisfaction of
predecessor logic
• Double Dependencies
– Are those that describe leads/lags on BOTH the
predecessor and successor activities
• Time & Volume Lags
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Project Conditions That Should be
Considered…
• Sometimes the activity network is conditional. For
example…
– If some event is scheduled during winter then a
group of activities will not be the same as they would
in Summer
– If some milestone will be met with a delay, certain
corrective actions will be taken
• In cases like this, the network should include options
that will be selected during project scheduling
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Scheduling and [Realistic] Results
• The schedule should have minimal possible durations
and should take into account all previously described
restrictions, including…
– Quantity of limited renewable resources available at
any given time
– Material and equipment availability at any given time
– Sufficient financing at any given time
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CPM Scheduling and Results
• Has a static, mathematical solution
• Assumes unlimited resources
• Usually results in unrealistic or unattainable schedules
• Leads to excessive manipulation of schedule data
• Leads to excessive explanation of methods, estimates
and performance
• Can be devalued by management if too much of the
above takes place- “what good is it?!”
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PM Software Failures
• Requires heavy maintenance and support
• Lacks reliable Heuristic capabilities and results
• Requires “guesswork” for resource “optimization”
• Create inaccurate float values and erroneous Critical
Paths
• Drives use of multiple software systems
• Leads management to focus on the wrong issues
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PM Software Failures
• MSP
– Uses predefined priority rules to level workloads
• P6
– Asks users to select priorities from a set of
predefined rules to level workloads
• These approaches DO NOT EVEN TRY to find the
optimal solution
– The chances of finding a better schedule can be
significantly improved with good heuristics
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PM Software Examples & Results
• Project “Test” consists of 4 activities and Finish
milestone, and uses two resources (A and B)
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PM Software Examples & Results
• The schedule created by MSP 2013 lasts 60 days and
total slacks are obviously wrong: activity 3 is shown as
critical though it has 19 days free float, activities 1 and 2
are critical (their delay will delay the project finish date)
but MS Project shows that both activities have 20 days
total slack.
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PM Software Examples & Results
• Let’s look at what happens if the last activity of this
schedule is one day longer:
• Now MSP selects the optimal sequence execution but
still shows the wrong total slack for activities 3 and 4
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PM Software Examples & Results
• …Which illustrates another problem:
– resource constrained schedules created by project
management software are not stable. Small changes
in activity durations may have huge impacts on the
sequence of activity execution and overall project
duration.
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PM Software Examples & Results
• A default resource constrained schedule created for this
project by P6 is as poor as the schedule created by MS
Project:
• Free Float of activity 2 in this schedule is… WHAT?!
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PM Software Examples & Results
• If we select OD as the main priority for resource leveling
then the schedule will be better, but the floats will
remain wrong:
• But… The best heuristics for one part of the project may
be the worst for another!
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PM Software Examples & Results
• Let’s look at the schedule created for the same project
by Spider Project:
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PM Software Examples & Results
• All activities are critical in the optimal schedule that was
automatically created
• Spider Project also shows resource dependencies
(activity 1 was delayed because a required resource
was busy on activity 3)
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Manual Leveling
• Several Problems
– It’s not likely that optimal or even near optimal
solutions will be found
– When the project is executed, some deviation from
the initial plan will certainly happen
– Creating soft dependencies is difficult, as can be
seen in the following example…
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Manual Leveling
• Example project “Soft”
– Consists of three independent activities of the same
duration
– Each activity requires the same resource (A)
– Only 2 units of resource A are available
– How do we create soft dependencies in this case?
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Manual Leveling
• It is not clear if activity 3 should follow activity 1 or
activity 2. After the finish of either of them, activity 3 may
be executed
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Manual Leveling
• If we will link activities 2 and 3, this link may delay the
project finish date if activity 1 is completed faster than
activity 2
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Manual Leveling
• Soft dependencies created for manual resource leveling
cannot replace a good software tool that evaluates
many options and selects the best order of activity
execution without creating artificial links
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Other Scheduling Options - Cost and
Material constrained scheduling
• Not only expenses but also incomes
• Not only material consumption, but also material
production and supply
• Space constraints are usually modeled as material
constraints
• Project management software needs to delay project
activities that require materials that are not available yet
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Other Scheduling Options - Advanced
Methods of Constrained Scheduling
• Shift work simulation
– Which shift will be ready when it’s time to start?
• Skill scheduling
– Used when there is a choice of skill-specific
resources
• Variable resource assignments
– Assigning a range of resource requirements will allow
maximum resource utilization
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Other Scheduling Options – Schedule
Stability
• Resource constrained schedules created in some
applications are not stable
– Small changes in durations of some activities may
create large changes in the project schedule,
including changes in the sequence of activity
execution
– Even if changes result in shorter schedules, they may
not be acceptable to stakeholders
• Stabilization is no less important than optimization;
Spider Project allows users to maintain activity
sequencing when recalculating an optimized schedule
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Conclusion
• Project schedules
– Must consider all existing constraints
– Should model real life work
• Resource constrained schedules created by different
packages for the same project may be different
• Manual leveling is not practical
• Schedule stabilization is just as important as
optimization
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Conclusion
• A Project Schedule should consider all existing
constraints including resource, supply, space, and
financing constraints and should maximize resource
utilization resulting in minimal project duration.
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©2012 Permission is granted to PMI for PMI® Marketplace use only.
Contact Information
Shane Archibald
Archibald Associates & Spider Project (USA)
(425) 241-3107
Session #9
“PMI” is a registered trade and service mark of the Project Management Institute, Inc.
©2012 Permission is granted to PMI for PMI® Marketplace use only.