Problems, Methods and Tools of Advanced Constrained Scheduling Session 9 Shane Archibald Archibald...

Problems, Methods and Tools of Advanced Constrained Scheduling

Session 9

Shane ArchibaldArchibald 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(R1

N = 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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• 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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• 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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• …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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• 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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• 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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• Let’s look at the schedule created for the same project by Spider Project:


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• 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.



Contact Information

Shane Archibald

Archibald Associates & Spider Project (USA)

(425) 241-3107

Session #9

Problems, Methods and Tools of Advanced Constrained Scheduling Session 9 Shane Archibald Archibald...

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