The Psychology of Quality and More
Activity Network: How to do it
How to do it
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
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.
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.
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.
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
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
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
And the big