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Dependency Determination and Types in CPM Scheduling

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In Critical Path Method (CPM) scheduling, determining task dependencies is fundamental to constructing an accurate project schedule. Dependencies define how tasks relate to one another and establish the sequence of activities. There are four primary types of dependencies in CPM scheduling: 1. **Finish-to-Start (FS)**: The most common dependency where a successor task cannot begin until its predecessor task has finished. For example, painting cannot start until surface preparation is complete. 2. **Start-to-Start (SS)**: A task cannot start until its predecessor has started. This allows two activities to begin simultaneously or with a delay defined by a lag. For instance, programming can start once design has started. 3. **Finish-to-Finish (FF)**: A task cannot finish until its predecessor has finished. This is useful when two activities must conclude together. For example, quality assurance must finish when testing concludes. 4. **Start-to-Finish (SF)**: The least common dependency where a task cannot finish until its predecessor has started. This might occur in scenarios like shift changes where the new shift must begin before the previous one can end. Understanding these dependencies is crucial for accurate schedule development. It ensures that tasks are sequenced logically and resources are allocated efficiently. Dependencies can be further classified as **mandatory**, **discretionary**, or **external**: - **Mandatory Dependencies**: Inherent in the nature of the work, such as legal or physical constraints. For example, concrete must cure before formwork is removed. - **Discretionary Dependencies**: Defined by the project team based on best practices or preferences. For instance, choosing to complete all design work before starting construction, even if some construction activities could technically begin earlier. - **External Dependencies**: Involve relationships between project tasks and external events or activities outside the project's control, such as regulatory approvals or delivery of materials from a supplier. Proper dependency determination helps identify the critical path accurately, assess project risks, and develop realistic schedules. It allows project managers to anticipate potential delays and implement mitigation strategies. Misidentifying dependencies can lead to scheduling conflicts, resource overallocation, and inaccurate timelines, adversely affecting project outcomes.

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Question 1

According to PMI scheduling standards, which dependency type best describes when an activity's completion allows the start of its successor, but the successor can also begin earlier with an overlap?

Correct Answer: Finish-to-Start with lead time

A Finish-to-Start (FS) relationship with lead time is the correct dependency type for this scenario. Here's why: In project scheduling, when an activity can start before its predecessor is fully complete, this represents a lead time (negative lag) in a Finish-to-Start relationship. This allows for overlapping of activities while maintaining the basic FS dependency. The other options are incorrect because: - Mandatory concurrent scheduling is not a standard PMI term and would not allow for optional overlap - Start-to-Start with partial completion requirements is incorrect because SS relationships deal with when activities start in relation to each other, not their finish points - Finish-to-Finish with early start modifications focuses on the completion relationship between activities, not the start of the successor activity A Finish-to-Start relationship with lead time is the standard way in PMI methodology to describe dependencies where activities can overlap but still maintain their basic finish-to-start relationship.

Question 2

In CPM scheduling, what determines a Finish-to-Start (FS) mandatory dependency between two construction activities?

Correct Answer: Technical requirements that prevent the second activity from starting until the first is complete

A Finish-to-Start (FS) mandatory dependency in CPM scheduling is determined by technical requirements and physical limitations that make it impossible for a successor activity to begin until its predecessor is complete. For example, concrete must fully cure before framing can begin, or a foundation must be completed before walls can be erected. These are physical, technical necessities - not preferences or optional guidelines. The other answers are incorrect because: Resource optimization strategies are discretionary dependencies, not mandatory ones. They can be adjusted based on resource availability. Contractual obligations may influence scheduling but do not create technical dependencies between activities. Best practices are guidelines that can be modified based on project circumstances, unlike mandatory dependencies which are based on physical or technical requirements.

Question 3

In CPM scheduling, which type of dependency would best reflect a requirement where an activity must maintain a constant time interval of 5 days ahead of its successor throughout their durations?

Correct Answer: Start-to-Start (SS) with a 5-day lag

The Start-to-Start (SS) relationship with a 5-day lag is the correct answer because it maintains a constant time interval between the start dates of two activities throughout their duration. This exactly matches the requirement where an activity must maintain a constant 5-day interval ahead of its successor. A Finish-to-Start (FS) relationship would only control the gap between the end of one activity and the start of another, not maintaining a constant interval during execution. A Start-to-Finish (SF) dependency would create an illogical relationship where the successor would need to finish before the predecessor starts, which doesn't meet the requirement. A Finish-to-Finish (FF) relationship would only maintain a constant interval between the finish dates of activities, not throughout their durations as required in the question.

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