The Psychology of Quality and More

A Toolbook for Quality Improvement and Problem Solving (contents)

Activity Network: How to do it

The Quality Toolbook > Activity Network> How to do it

When to use it | How to understand it | Example | How to do it | Practical variations

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How to do it

1. Define the key objective of the plan, for example, 'to lay all paving stones in a street to a given pattern'. This forms the basic boundary of the project and lets you identify when the job is done.

 
2. Identify other constraints which may affect the actual planned actions. These typically will be around work, time and cost. For example, where the job must be done within a week, within a set budget, using available manpower, to government standards, and with minimum noise and disruption to local residents.

 
3. Identify the actual tasks that need to be done. This may be done through the use of a Tree Diagram (in this case, only the bottom-level tasks will be used in the Activity Network).

Write a short description of each task onto 3" x 5" cards. Also make space for earliest and latest start and finish dates.

 



Fig. 1. Writing the card

 

4. Write on each card the time that the task will take. Other information may also be included, such as the person (or persons) who will perform the task, the extra tools or resources required, etc. At this time, the person's name may not be known, but the required skill level should be known as this might affect the time estimate.

When writing in the time, try to use the same units for each task. For example, put them all in hours or all in days. This will make calculations easier.

The task durations will normally be within an order of magnitude of one another. Thus if most tasks are several hours long, but there is one task with a duration of three weeks, then this may well be an indication that this task should be broken down further.
 

5. Start from the beginning, and ask which tasks must be done first. Place these cards to the left of the working area. If there is more than one, place them spaced out one above the other.

 
6. For each task just placed, find the task cards which must immediately follow and place these to the right.

   If a task should start part-way through another task, then break the second task down into two or more tasks to enable clear start and finish links to be made.

   If the cards becomes squashed up or it is not clear from the positioning which card follows which, take a little time to rearrange them so the correct sequence is clear and there is space for subsequent cards to be added.
    

   

   Fig. 2. Positioning the cards
    

7. Repeat step 6 until there are no more task cards to place.

If the sequence of tasks is not clear, it can be easier to start with a central, well-understood task, and identify tasks which must go before and after it. Another strategy is to start at the end and work back to the start by asking of each task, 'What task must be done before I can do this?'.

Sometimes tasks may be found that should be placed in a part of the diagram that has already been laid out. If this happens, simply rearrange the diagram to fit in the new card.

If new tasks are identified during this process, just write up a new card.
 

8. Complete the links between tasks by drawing lines in the work area between cards. To do this, the cards need to be quite firmly attached to a work area that can be drawn on, such as a whiteboard or large sheet of paper.

When drawing the links, use arrows to indicate which task follow which. The order of tasks is made clearer if the tasks generally flow from left to right.

The early and late times for each task can now be calculated and the critical path identified.
 

9. Starting with the tasks at the beginning of the diagram, complete the early start and early finish for each task in turn, following the arrows to the next task. A task cannot be completed until all of its predecessors have been completed.

The early start of a task is the same as the early finish of the preceding task. If there is more than one predecessor task, then there are several possible early start figures. Select the largest of these.

The early finish for each task is simply the early start plus the duration of the task. The final calculation is for the earliest completion time for the project. This is calculated in the same way as the early start date.

 



Fig. 3. Calculating late start and late finish
 

10. Starting with the tasks at the end of the diagram, complete the late start and late finish for each task in turn, following the arrows in the reverse direction to the previous task. A task cannot be completed until all of its successors have been completed.

The late finish is the same as the late start of the succeeding task (for the final tasks in the project, this is equal to the earliest completion date, calculated in step 9). If there is more than one successor task, then there are several possible late figures. Select the smallest of these.

The late start for each task is simply the late finish minus the duration of the task. The final calculation is for the earliest completion time for the project. This is calculated in the same way as the early start date.
 



Fig. 4. Calculating backward

 

11. Calculate the slack time for each task as the difference between the early and late times.

Also identify the route through the diagram where the slack time on each task through the route is zero. This is called the critical path, as any slippage in these tasks will affect the overall project completion date.

 



Fig. 5. Calculating slack time

 

12. Evaluate and act upon the results, including checking that the final plan meets any constraints identified in step 2. Actions may include:

• Reducing the total slack in the project by rearranging tasks.
• Preventing people from having to work overtime by allocation, reallocation and task rearrangement.
• Identifying risky parts of the plan and reducing risks by reallocation or rearrangement.
• Recalculating the early and late times to find the effect of the above actions on the critical path, the project completion time and the slack.
• Rebudgeting to account for the effects of rearrangement or allocation.

13. Start the tasks, using the diagram to help manage the project. Management actions may include:

• Substituting actual durations of tasks into the diagram.
• Re-estimating future task durations, using known task durations.
• Adding, modifying or removing tasks.
• Rearranging the resources used.
• Recalculating the early and late times to find the effect of the above actions.

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