The first step in making an Activity Network Diagram is to compile a list of all the tasks that must be completed for the overall project. These tasks must then be arranged in a chronological order. Depending on the project, you might be able to work on some tasks simultaneously. Some tasks will probably depend on the prior completion of other tasks. For instance, if you were painting a house, you would not be able to begin painting until certain jobs (such as sanding the wood, treating the wood, and purchasing the paint) were completed. However, you might be able to purchase the paint and treat the wood at the same time. All the tasks will be placed in a progressing line. Jobs that can be done simultaneously can be placed on parallel paths, whereas jobs that are dependent should be placed in a chronological line of the activity network diagram.
You will need to know how long each job will take. Be sure to get a realistic estimate. Once you know how long each individual task will take, you will know how long the entire project will take. In addition, this method will let you see where you can afford delays. For instance, suppose that it takes 1 day to sand the wood and 3 days to treat the wood. Suppose that it only takes 1/2 day to buy the paint. The painting cannot begin until all three tasks are completed. Therefore, the paint purchaser does not need to buy the paint right away - she has a four-day period in which to do a 1/2 day job. In the 7MP jargon, we say that the purchaser has a 3 1/2 day slack time. However, if the workers responsible for the sanding and treatment do not complete their jobs within the allotted time, the entire project will be delayed. We say that these tasks lie on the critical path to the completion of this project. There is, by definition, no slack time on the critical path. .
Note: There are several types of Activity Network Diagram tools. This discussion assumes the Activity on Node type, meaning that each task is represented by a symbol. Arrows showing the sequential ordering of the tasks connect the symbols. Each sequentially ordered task should be completed before moving on to the next task. Tasks that may be done simultaneously should be represented on parallel paths.
ES = Earliest Starting Time, the time at which all predecessor jobs are completed.
EF = Earliest Finish Time, the time at which the job will be completed if it starts at the earliest possible starting time (ES).
LS = Latest Start Time, the latest time at which the job can start without delaying the project completion. On the critical path, the ES and the LS will always be the same number. Otherwise, the difference between the two will be the slack time. In the painting example above, the LS for paint purchasing would be 3.5. If the purchaser does not begin the task by day 3.5, the painters will have to wait for the paint and the project completion is delayed.
LF = Latest Finish Time, the time at which the job will be completed if it starts at the latest allowable start time (LS).
EF - ES = LF - LS = the duration of that particular job.