Key Takeaways
- PDM (Precedence Diagramming Method) uses four relationship types: Finish-to-Start (most common), Start-to-Start, Finish-to-Finish, and Start-to-Finish (rare)
- Critical Path Method (CPM) identifies the longest path through the network, determining minimum project duration with zero float
- Float (slack) is calculated as LS - ES or LF - EF, representing how long an activity can be delayed without impacting the project end date
- Crashing adds resources to critical path activities to reduce duration at the lowest incremental cost per time unit saved
- Fast-tracking performs sequential activities in parallel, reducing duration but increasing risk of rework
Schedule Management
Schedule management involves defining activities, sequencing them logically, estimating durations, and developing and controlling the project schedule. In predictive projects, the schedule is established during planning and managed through formal change control.
Schedule Management Processes
| Process | Purpose | Key Output |
|---|---|---|
| Plan Schedule Management | Define policies and procedures | Schedule Management Plan |
| Define Activities | Break WBS into schedulable activities | Activity List, Milestone List |
| Sequence Activities | Identify dependencies between activities | Network Diagram |
| Estimate Activity Durations | Determine time needed for each activity | Duration Estimates |
| Develop Schedule | Create the project schedule | Schedule Baseline |
| Control Schedule | Monitor status and manage changes | Work Performance Information |
Precedence Diagramming Method (PDM)
PDM is the most common method for creating network diagrams in project scheduling. Activities are represented as nodes (boxes), and dependencies are shown as arrows connecting them.
Four Types of Logical Relationships
| Relationship | Notation | Description | Example |
|---|---|---|---|
| Finish-to-Start (FS) | FS | Successor cannot start until predecessor finishes | Foundation must finish before walls can start |
| Start-to-Start (SS) | SS | Successor cannot start until predecessor starts | Testing can start when development starts |
| Finish-to-Finish (FF) | FF | Successor cannot finish until predecessor finishes | Documentation must finish when coding finishes |
| Start-to-Finish (SF) | SF | Successor cannot finish until predecessor starts | Night shift ends when day shift starts |
Relationship Usage
- Finish-to-Start (FS) is the most common relationship (approximately 90% of dependencies)
- Start-to-Finish (SF) is the least common and rarely used
Leads and Lags
| Term | Definition | Example |
|---|---|---|
| Lead | Amount of time a successor can start BEFORE predecessor finishes | FS -2 days: Start 2 days before predecessor finishes |
| Lag | Amount of time a successor must wait AFTER predecessor finishes | FS +3 days: Wait 3 days after predecessor finishes |
Dependency Types
Understanding why activities are dependent helps with schedule optimization:
| Dependency Type | Description | Flexibility |
|---|---|---|
| Mandatory (Hard Logic) | Required by nature of work | Cannot be changed |
| Discretionary (Soft Logic) | Based on best practices or preferences | Can be modified |
| External | Relationship to non-project activities | Outside project control |
| Internal | Within project team's control | Team can adjust |
Critical Path Method (CPM)
The Critical Path Method is the most important schedule analysis technique. It identifies the longest path through the network diagram, which determines the minimum project duration.
CPM Calculations
| Calculation | Formula | Purpose |
|---|---|---|
| Forward Pass | ES of successor = EF of predecessor | Calculate earliest start and finish dates |
| EF = ES + Duration | ||
| Backward Pass | LS = LF - Duration | Calculate latest start and finish dates |
| LF of predecessor = LS of successor | ||
| Total Float | Float = LS - ES = LF - EF | Time activity can slip without delaying project |
| Free Float | EF - ES of successor | Time activity can slip without delaying successor |
Critical Path Characteristics
- The critical path is the longest path through the network
- Activities on the critical path have zero float
- Any delay on the critical path delays the entire project
- A project can have multiple critical paths
- The critical path can change during project execution
CPM Calculation Example
Consider this network with durations in days:
| Activity | Duration | Predecessor |
|---|---|---|
| A | 3 | None |
| B | 4 | A |
| C | 2 | A |
| D | 5 | B |
| E | 3 | C |
| F | 2 | D, E |
Forward Pass:
- A: ES=0, EF=3
- B: ES=3, EF=7
- C: ES=3, EF=5
- D: ES=7, EF=12
- E: ES=5, EF=8
- F: ES=12 (max of D and E finish), EF=14
Project Duration: 14 days
Critical Path: A → B → D → F
Float Analysis
Float (also called slack) represents schedule flexibility:
Types of Float
| Type | Definition | Impact |
|---|---|---|
| Total Float | Time an activity can slip without delaying project end | Project impact |
| Free Float | Time an activity can slip without delaying any successor | Activity impact |
| Project Float | Difference between imposed deadline and CPM end date | Overall buffer |
Negative Float
Negative float indicates the project is behind schedule:
- Schedule shows project will finish AFTER the required deadline
- Requires schedule compression to meet the deadline
- Common cause: Imposed deadlines, late starts, extended activities
Schedule Compression Techniques
When the schedule needs to be shortened, two primary techniques are available:
Crashing
Crashing involves adding resources to critical path activities to reduce their duration:
| Aspect | Description |
|---|---|
| How It Works | Add people, equipment, or overtime |
| Selection Criteria | Choose activities with lowest crash cost per day |
| Limitation | Only works on critical path activities |
| Risk | Increases cost; diminishing returns apply |
Crash Cost Calculation
Crash Cost per Day = (Crash Cost - Normal Cost) / (Normal Duration - Crash Duration)
| Activity | Normal Duration | Crash Duration | Normal Cost | Crash Cost | Crash Cost/Day |
|---|---|---|---|---|---|
| A | 10 days | 8 days | $10,000 | $14,000 | $2,000/day |
| B | 8 days | 6 days | $8,000 | $14,000 | $3,000/day |
| C | 12 days | 9 days | $15,000 | $21,000 | $2,000/day |
Decision: Crash Activity A or C first (lowest cost per day saved)
Fast-Tracking
Fast-tracking involves performing sequential activities in parallel:
| Aspect | Description |
|---|---|
| How It Works | Overlap activities that were planned sequentially |
| Requirement | Activities must be able to run in parallel |
| Limitation | Only works with discretionary dependencies |
| Risk | Increases risk of rework; quality may suffer |
Crashing vs. Fast-Tracking
| Factor | Crashing | Fast-Tracking |
|---|---|---|
| Cost Impact | Increases cost | May or may not increase cost |
| Risk Impact | Lower risk | Higher risk (rework) |
| Resource Impact | Requires additional resources | Uses existing resources differently |
| Best Use | When budget is available | When time is critical |
Schedule Baseline
The Schedule Baseline is the approved version of the schedule that provides the basis for performance measurement:
Components
- Approved start and finish dates for all activities
- Milestone dates
- Resource assignments
- Schedule reserve (if applicable)
Changes to the baseline require formal integrated change control.
Key Takeaways
- PDM uses four relationship types, with Finish-to-Start being most common
- The Critical Path is the longest path and determines minimum project duration
- Float = LS - ES = LF - EF measures schedule flexibility
- Crashing adds resources to reduce duration at additional cost
- Fast-tracking performs sequential activities in parallel, increasing risk
- The Schedule Baseline is the approved schedule used for performance measurement
An activity has ES=5, EF=10, LS=8, and LF=13. What is the total float for this activity?
A project manager needs to shorten the schedule and has budget available. Which technique should be applied first?
Which logical relationship indicates that Activity B cannot START until Activity A FINISHES?