4) Análisis de resultados
4.2 El nivel de Estudios
A
Activity Network Diagram (AND)—7M Tool
What Question(s) Does the Tool or Technique Answer?
What is the most efficient way to complete this process or project?
An AND helps you to
• Manage the timing of activities and overall completion of a project or process
• Graphically organize process steps into the most efficient sequence
• Show the most critical path and any parallel paths
• Evaluate and reorganize the step sequence. It identifies any simulta-neous tasks and tasks that will take the longest to complete
• Identify any slack time—that is, the amount of time a non-critical path task can be delayed without delaying the project
• Manage resources and understand upstream and downstream dependencies
Alternative Names and Variations This tool is also known as
• Activity chart
• Activity-on-Arrow (AOA)
• Arrow diagram or Arrow Diagramming Method (ADM)
• Activity on Node (AON)
• Critical Path Method (CPM) or CPM chart
• Network diagram
• Node diagram
• Precedence Diagram (PDM) Variations on the tool include
• Program Evaluation and Review Technique (PERT) chart. (See Also
“PERT (Program Evaluation and Review Technique) Chart,” p. 453)
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When Best to Use the Tool or Technique
The Activity Network diagram (AND) is an important Project Management tool. It helps to sequence the steps of the project into the most efficient order such that some steps are completed simultaneously. Most project managers will use the AND technique with the project team prior to devel-oping the project schedule that commits the team to a target project com-pletion date. The AND technique is helpful when managing a project whose deadline is critical and for which any delay has significant conse-quences—and conversely whose acceleration could yield great benefits.
Another application of the AND tool is from the perspective of the process of interest. Similar to a process map, the AND technique evalu-ates the existing steps in a process to identify the critical path and any parallel activities.
The critical path is comprised of a set of tasks that if delayed will delay the outcome of a process or the completion of the project. Even if one crit-ical path activity is delayed, the timing of the overall process or project is negatively impacted. As a result, the critical path identifies the least flexi-ble activities critical to maintaining the schedule and the float activities.
The critical path also calculates slack time associated with the non-critical path activities to understand the flexibility of their start and finish with-out impacting the overall schedule determined by the critical path.
Brief Description
The Activity Network diagram (AND) is a simple graphical representa-tion of the process or project steps. It depicts which steps must be com-pleted when to complete a project and in what sequence. It can be done either manually or aided by computer software.
A typical AND includes the following symbols, shown in Figure A-1:
• Rectangles, called nodes, represent the process or project’s activities (hence, the technique is sometimes called Activity on Node).
• Arrows connect the activities and show dependencies between two activities.
There are four types of dependencies. These dependencies describe the logical relationship between two activities. Sometimes they also are referred to as precedence relationships; hence, the technique is sometimes called the Precedence Diagramming Method (PDM). Figure A-2 illustrates the various types of dependencies.
A
Activity A Activity B
“From” “To”
Figure A-1:AND Using Activity on Node Symbols
ptg6842824 1. Finish-to-Start—Where the “from” activity must finish before the
“to” activity can start. This is the most common dependency.
2. Finish-to-Finish—Where the “from” activity must finish before the
“to” activity can finish.
3. Start-to-Start—Where the “from” activity must start before the “to”
activity can start.
4. Start-to-Finish—Where the “from” activity must start before the
“to” activity can finish. This type of dependency is rarely used and is usually only used in the engineering discipline.
Activity Network Diagram (AND)—7M Tool 129
Activity A Activity B 5-day duration 7-day duration
Finish-Start
A B
12-day total duration
Activity A
12-day total duration Activity A 7-day total duration
Activity A 7-day total duration
Figure A-2:AND Four Different Dependencies
A less common set of symbols includes circles rather than rectangles, as illustrated in Figure A-3. However, this less common approach has the added nuance of using the arrows to represent the activities, rather than the shape, which is reversed from the more traditional approach. Both methods work equally well, and it is a matter of preference as to which technique to use.
• Arrows represent activities (hence, the technique is sometimes called Activity-on-Arrow). The tail of the arrow represents the beginning of the activity, and the head of the arrow represents its completion.
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• Connected at nodes showing the dependencies between two activities.
The nodes often contain lettering in the center of the circle to identify it.
As shown in Figure A-3, the “Prepare Spec” task also may be referred to as the AB activity.
The Activity on Arrow technique, by design, uses the Finish-to-Start dependency.
The AND technique is a member of the 7M Tools, attributed in part to Dr.
Shewhart, as seven management tools, sometimes referred to as the 7MP (or seven management and planning tools). These 7M Tools make up the set of traditional quality tools used to analyze quantitative data. The 7M Toolset includes 1) Activity Network diagrams or Arrow diagrams, 2) Affinity diagrams, 3) Interrelationship digraphs or Relations diagrams, 4) Matrix diagrams, 5) Prioritization matrices, often replacing the more com-plex Matrix data analysis, 6) Process decision program charts (PDPC), and 7) Tree diagrams. The Quality Toolbox by Nancy Tague presents the 7M Tools ranked from those used for abstract analysis to detailed plan-ning: Affinity diagram, Relations diagram, Tree diagram, Matrix diagram, Matrix data analysis (commonly replaced by a more simple Prioritization matrix), Arrow diagram, and Process Decision Program Chart (PDPC).
Technically speaking, the Critical Path Method (CPM) and the Pro-gram Evaluation and Review Technique (PERT) chart are estimating tech-niques applied to a completed Activity Network diagram, rather than a different type of AND. A critical path identifies the project path with the longest duration, which also determines the shortest time to complete the project. Hence, the critical path lacks any slack time; it represents the min-imum amount of time to complete the project. Slack time is the amount of time a task can be delayed without delaying a project. In contrast, if par-allel paths exist, they will contain slack time or a shorter duration than the critical path. How to calculate the critical path is described in this encyclopedia, in the PERT technique section under the Ps. (See Also
“PERT (Program Evaluation and Review Technique) Chart,” p. 453)
How to Use the Tool or Technique
This technique can be done individually or in a small group. When using the simple approach to develop an AND, the procedure involves the fol-lowing steps:
Step 1. Identify the topic and then collect any data about the task.
Examples include
Figure A-3: AND Using Activity-on-Arrow Symbols
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c. Procedure manuals
Step 2. List the necessary tasks.One technique used is to create a mas-ter list and then to put one activity on either index cards or sticky notes to make it easy to sequence the tasks.
Activity Network Diagram (AND)—7M Tool 131
Warning
Be sure you only assign one activity step per index card or sticky note!
Step 3. Place the tasks into a logical order sequence.Identify which tasks must precede others, and arrange them accordingly. If some tasks can occur simultaneously, put them on the same vertical plane (parallel to one another). If using sticky notes, follow the same procedure and place the activities on a flip-chart sheet or section of the wall.
Continue this step until all the individual steps are placed in a logical flow. Be sure to leave space in-between each step to allow additional notes to be added.
Step 4. Add times to the steps(usually above the activity card or note).
Step 5. Determine the critical path.
a. Identify all the possible path combinations from start to finish.
b. Sum the times of each activity in each of the possible path combinations and record the path and its respective duration.
c. Identify the longest path and highlight by bolding (high-lighting or darkening) the path arrows—this is the critical path.
d. Calculate the Earliest Start (ES) times and Earliest Finish (EF) times based on how long the preceding task(s) take.
Hence, the first activity has zero ES time, and its EF time equals the total duration for the task. Preceding tasks have a compounding (or cumulative) effect on subsequent tasks’ ES and EF times.
i. ES = the latest EF from the preceding task ii. EF = the ES time minus the actual task time
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e. Calculate the Latest Start (LS) times and Latest Finish (LF) times by starting at the end of the network (project or process completion) and working backward toward the start activity.
i. LF = the shortest LS of the subsequent task ii. LS = the LF time minus the actual task time
f. Calculate the slack times for each task and for the project.
Total slack is the time an activity can be delayed without affecting the project schedule. Free slack is the time an activity can be delayed without affecting the early start of any subsequent task.
i. Total slack = LS minus ES = LF minus EF ii. Free slack = the earliest ES of all subsequent tasks
minus EF
Step 6. Examine and adjust the diagram as needed.
How to Analyze and Apply the Tool’s Output
After the Activity Network diagram is complete, use the output to
• Reveal the interdependencies of activities.
• Determine what elements are critical to meeting the completion date (the critical path), including necessary resources and time requirements.
• Determine projected completion date.
• Facilitate “what if” discussions (or alternatives) as to what could be shortened or rearranged; consider any trade-offs.
• Highlight critical activities that will require monitoring and provide a baseline to evaluate the actual process or project performance.
Acquire a tool for planning and predicting.
• Help to manage uncertainties, particularly where slack time exists.
Examples
Next is an example of an AND depicting six activities with a Finish-Start dependency. The list of activities includes the following:
• Activity A @ 3-day duration
• Activity B @ 5-day duration A
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• Activity C @ 7-day duration
• Activity D @ 4-day duration
• Activity E @ 9-day duration
• Activity F @ 3-day duration This 6-step
sce-nario with its crit-ical path and slack time calcula-tions can be found in Figure A-4.
This example’s
The longest path is A-B-C-E-F, at 27 days; therefore, it is the critical path.
The calculations for the Earliest and Latest times are found in Figure A-4, and the formulas are as follows:
• ES = the latest EF from the preceding task
• EF = the ES time—the actual task time
• LF = the shortest LS of the subsequent task
• LS = the LF time—the actual task time The calculations for the slack times are
• Total slack = LS—ES = LF—EF;
• Free slack = the earliest ES of all subsequent tasks—EF
Activity Network Diagram (AND)—7M Tool 133
Activity A
Figure A-4:6-Step AND Example
Note
There is zero slack time along the critical path.
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• No activity can start
no meaning; it simply represents logical precedence.
• Each activity must have a unique identifier (a number or alpha);
duplicates are not permitted.
• If an Arrow Diagramming method is used, only finish-to-start dependencies are appropriate.
• AOA may use dummy activities (with zero time durations) to help com-plete the logic in the network dia-gram by showing the flow of
activities. Dummy activities are iden-tified with hashed or dotted arrows, as shown in Figure A-6. They typi-cally show relationships between activities that have more than one
predecessor by keeping the sequence correct.
• Non-sequential steps, such as loops or conditional branches using deci-sion-diamonds are not used in any of the AND techniques. These A
Hints and Tips
A summary activity, called a hammock, represents a group of related activities as one to show the lapsed time for the collection of tasks.
The hammock spans between two activities that may be separated by several other activities, as illustrated in Figure A-5.
Activity A Activity B Activity D
Activity H
Activity E
Activity C Activity F
Figure A-5: AND Hammock (as Activity H)
A B C
Figure A-6:Activity-on-Arrow with Dummy Activity
ptg6842824 special conditions are found in process mapping and more
special-ized diagramming techniques called Graphical Evaluation and Review Technique (GERT) and Systems Dynamics modeling.
Supporting or Linked Tools
Supporting tools that might provide input when developing an AND include
• VOC Data Gathering Tools and Techniques, such as surveys and interviews (See Also“Voice of Customer Gathering Techniques,”
p. 737)
• Written reports
• Brainstorming sessions (See Also“Brainstorming Technique,” p. 168) A completed AND provides input to tools such as
• Tree diagram and matrix (See Also“Tree Diagrams,” p. 712)
• Fishbone (See Also“Cause-and-Effect Diagrams,” p. 173)
• Simple matrix (See Also“Matrix Diagrams,” p. 399)
Figure A-7 illustrates the link between the AND and its related tools and techniques.
Activity Network Diagram (AND)—7M Tool 135
Activity Network
Figure A-7: Activity Network Diagram Tool Linkage
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