BMATWT 353 - Business of Building

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Project Scheduling

Resource: On-line Readings, CPM and PERT

Lecture objectives

1. Define Scheduling
2. Cost considerations in making scheduling decisions
3. Costs of changes to schedule
4. Importance of "time of delivery" promises
5. The Learning Curve
6. Gantt Charts
7. Critical Path Method Scheduling and PERT

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1. Definitions

- Scheduling - Provides a sequential order and time when products, information, or people move through processes.

A "good" Schedule should assist the timely completion of a project.

Milestones - Major events, often tied to "deliverables" and payment schedules.

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2. Cost Considerations in Scheduling Decisions

1) Minimize Costs of starting or completing a customer's order too early OR too late.

• Inventory carrying costs - space, spoilage, damage, theft, obsolescence, crowding
• Contract delivery penalties - Late completion penalties $ per day.
• Reputation

2) Minimize cost of non-productive resources

• Excess plant space
• Idle equipment
• Idle employees (wages AND morale demotivator)

3) Minimize cost of maintaining the Scheduling system itself.

• Take a number - deli counter
• Dentists Office - schedule 6 months in advance
• Automobile Manufacturing Plant - Just-in-time supply, order custom vehicles
• Big Dig - Multi-year, multi-billion dollar construction project

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3. What are some of the costs involved with schedule (or production volume) changes?

1) Unstable employment

• Hiring costs - recruiting, interviewing, testing potential recruits
• Training costs
• Firing Costs - unemployment insurance

2) Unstable supplier relations

• increasing orders
• cancelling orders
• delaying orders

Business loves STABILITY.

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4. Importance of "time of delivery" promises

• You can have any two of the following: Price, Quality or Service
• You can measure Service in terms of "time of delivery"
• Sometimes, delivery on schedule, can be as important, or more important than price or even quality.
• To improve ANYTHING over time, you need to MEASURE it.
• Delivery time measurements:
  • Percentage - on-time delivery (for the project as a whole, or for suppliers, or for subcontractors) Think of airline service statistics.
  • Elapsed time to fulfill order
• Importance of "time of delivery" varies across firms, individuals, and cultures

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5. The Learning Curve and Scheduling Rules

• The more we do a task, the better we get at it.
• This parallels the concept of specialization.
• Shape of the Learning Curve is typically downward sloping on a logarithmic progression.

 

• Priority rules for real-time scheduling: None of these appears to work better than any other on a consistent basis.
• First-come first-served.
• Earliest due date.
• Largest number of operations remaining.
• Shortest set-up time
• Employee choice

 

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6. Gantt Charts

Bar chart - graphical display

Tasks along left hand side (row by row)

Time along X-axis (weeks or months)

Gantt chart shortcomings:

Usually can't show task interdependencies

 

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7. Critical Path Method Scheduling and PERT

Developed in 1957 by DuPont to manage plant maintenance shut-down schedules.

CPM - Critical Path Method

Benefits:

• Graphic view of the overall project
• Predicts the time to completion for the project
• Shows the most critical activities that are controlling the projects completion time.

Elements of CPM

Network Diagram

• Nodes: Depict specific activities
• Lines: Depict beginning or end requirements for an activity

Steps in CPM Planning:

1. Identify and list each activity in the project.
2. Determine the sequence of the activities. Can a task be conducted in "Parallel" with others? Or, is it a "Sequential" or "serial" task, a task that is dependent on an earlier task being completed before it can be begun.
3. Draw the network diagram.
4. Estimate the time to complete each activity.
5. Identify the critical path through the network (which path takes the longest total time?)
6. Update the CPM diagram when changes occur.

 

CPM Times

• ES - Earliest start time
• EF - Earliest finish time
• LF - Latest finish time
• LS - Latest start time

Slack time = (LS - ES) or = (LF - EF)

On the critical path there is NO slack time.

CPM Limitations

Developed for routine operations, must know with some precision the actual activity times.

 

 

PERT - Program Evaluation and Review Technique

Developed for the Polaris missile submarine project.

Similar to CPM but adds uncertainty of activity completion times and statistical analysis of probabilities of time of completion.

PERT Times

• Optimistic time
• Most likely time
• Pessimistic time