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Precedence Diagramming Method (PDM)

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The Precedence Diagramming Method (PDM) is a fundamental tool used in project scheduling for visually depicting the relationships between activities. In PDM, activities are represented as nodes (commonly rectangles), and dependencies between them are shown as arrows connecting these nodes. This method allows project managers to identify the sequence in which activities must be performed, establish the logical relationships, and determine the critical path of the project. There are four types of dependencies utilized in PDM to define the relationships between activities: 1. **Finish-to-Start (FS):** The successor activity cannot start until the predecessor activity finishes. This is the most common type of dependency. 2. **Finish-to-Finish (FF):** The successor activity cannot finish until the predecessor activity finishes. 3. **Start-to-Start (SS):** The successor activity cannot start until the predecessor activity starts. 4. **Start-to-Finish (SF):** The successor activity cannot finish until the predecessor activity starts. This type is rarely used. By employing PDM, project managers can effectively plan and control the project schedule. It facilitates the identification of potential scheduling issues early in the planning process, such as bottlenecks or resource conflicts. Additionally, PDM supports the application of critical path method (CPM) analysis to determine the minimum project duration and identify activities that have flexibility in their scheduling (float). Understanding and utilizing PDM is essential for efficient project scheduling and for communicating the project plan to stakeholders.

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Question 1

In a Precedence Diagramming Method (PDM), if there are three activities A, B, and C where B has a Start-to-Start relationship with A and a Finish-to-Finish relationship with C, what best describes this activity network?

Correct Answer: Activity B can start with A and must finish when C finishes

In PDM, dependencies define the relationships between activities. When we have a Start-to-Start (SS) relationship between activities A and B, it means B can start when A starts. When we have a Finish-to-Finish (FF) relationship between B and C, it means B must finish when C finishes. The key points are: 1. SS relationship: Activity B can begin when A begins 2. FF relationship: Activity B must finish when C finishes The other options are incorrect because: - B does not need to complete before C starts - they have a FF relationship, not a Finish-to-Start - B is not running completely parallel as it has specific dependencies with both A and C - B does not need to wait for A to finish - it only needs A to start (SS relationship) The correct relationship shows that B can start with A (SS) and must finish when C finishes (FF), which accurately describes both dependencies in the network.

Question 2

In a Precedence Diagramming Method (PDM), when Task A has a positive lag of 2 days and a negative lag of 3 days with two different successor activities, what does this indicate?

Correct Answer: Task A's second successor starts 3 days before A ends, while the first successor starts 2 days after A ends

In PDM (Precedence Diagramming Method), lag represents the amount of time between a predecessor and successor activity. A positive lag of 2 days means that after Task A ends, there is a waiting period of 2 days before the first successor activity can begin. A negative lag (or lead) of 3 days means that the second successor activity can start 3 days before Task A finishes. These types of relationships are common in construction and manufacturing projects where work can overlap or requires mandatory waiting periods. The other answers are incorrect because: - Resource utilization and schedule compression are not related to lag relationships in PDM - Mandatory waiting periods before starting tasks are not indicated by positive/negative lags - Baseline schedule adjustments are not determined by lag relationships Lag relationships specifically define the timing between the end of one task and the start of another task, which is precisely what the correct answer describes.

Question 3

In the Precedence Diagramming Method (PDM), if Activity A and Activity B can start at the same time, but Activity B must finish 3 days after Activity A finishes, what relationship type and lag combination should be used?

Correct Answer: Start-to-Start (SS) with Finish-to-Finish (FF) +3 days lag

The correct answer is Start-to-Start (SS) with Finish-to-Finish (FF) +3 days lag. Let's analyze the requirements: 1. Activities A and B can start at the same time - This requires a Start-to-Start (SS) relationship 2. Activity B must finish 3 days after Activity A finishes - This requires a Finish-to-Finish (FF) relationship with +3 days lag This is a case where dual relationships are needed to properly constrain the activities. The SS relationship ensures both activities can start simultaneously, while the FF+3 ensures the proper completion sequence. The other options are incorrect because: - Using SS with FF -3 days and buffer would create an improper completion sequence - An SS relationship with mandatory completion delay would not properly control the finish relationship between the activities - Using FS with SF relationships would not allow the activities to start simultaneously, which is a key requirement

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