| The activity network diagram is a method of displaying | | | | 4. Assign time duration to each task or job. Write it on |
| the timelines of all the various subtasks that are | | | | the bottom half of the card. Since you will be adding |
| involved in any project. By doing this, the total task | | | | these times, keep the numbers consistent. For instance, |
| duration and the earliest and latest start and finish | | | | do not have some cards showing days for completion, |
| times for each task are also calculated and displayed. | | | | some showing hours and some showing minutes. |
| In addition to showing which subtasks are critical to | | | | Select the Lowest Common Denominator. |
| on-time task completion, the activity network diagram | | | | 5. Calculate the shortest possible time within which the |
| can help determine where extra effort to speed a | | | | overall task can be completed by adding the times of |
| subtask will have the greatest payoff to overall speed. | | | | each subtask to find the path of the longest cumulative |
| The activity network diagram has had a relatively long | | | | duration. This is the critical path. Knowing the critical |
| history, dating back to the 1930s. In the 1950s, the | | | | path is important because this will tell if the time |
| technique emerged as the Program Evaluation | | | | objectives of the project are attainable. The critical |
| Research Technique (PERT) and as the Critical Path | | | | path identifies those jobs or tasks that have no slack. |
| Method (CPM). There are several ways to represent | | | | Each must be done on time if the project is to stay on |
| the output of the PERT/CPM process. | | | | schedule. The critical path also identifies targets for |
| The method called the activity-on-arrow or, more | | | | improvement to increase speed. (If tasks on the critical |
| simply, the arrow diagram will be reviewed in this | | | | path can be sped up, the overall time to complete the |
| article. An arrow diagram treats numbered nodes as | | | | project may be able to be shortened. Remember, |
| instantaneous stop/start points for activities. The | | | | however, that if a task on the critical path is sped up, a |
| activities themselves are considered to take place on | | | | different path may become the critical path.) |
| the arrows connecting the nodes. | | | | 6. Calculate the earliest starting and finishing times and |
| What can it do for you? | | | | the latest starting and finishing times for each job or |
| An activity network diagram can show you which | | | | subtask in the project. Begin at the start of the |
| activities or which series of activities is critical to the | | | | diagram. The earliest start time for each job is the |
| timing of a more complex collection of interactive | | | | cumulative duration of all the previous jobs on that |
| activities. This can be very helpful in deciding where | | | | path. The earliest finish time is the earliest start time |
| and when to apply extra energy to keep projects on | | | | plus the duration of that task. Repeat this process for |
| time. Creating an activity network diagram is | | | | each job on each path until you reach the finish point. |
| time-consuming, however, so you should consider | | | | Next calculate the latest start and finish times. Begin |
| these questions before you decide to create one: | | | | with the earliest finish time at the end of the diagram. |
| 1. Is the task a complex one with simultaneous paths | | | | To calculate the slack time for any job or task, |
| that must be coordinated? Creating a diagram for a | | | | subtract the earliest start time from the latest start |
| relatively simple task may be a waste of time. | | | | time. All of the jobs on the critical path, by definition, will |
| 2. Are the durations of the subtasks known with | | | | have zero slack time. Remember that slack time is |
| relative certainty? If the actual timing of events is | | | | dependent on the time of completion of the previous |
| markedly different from diagram times, it will have little | | | | job or task. If some of the slack time in a path other |
| value, and people will dismiss the diagram as a useless | | | | than the critical path is used in an early task, the slack |
| exercise. | | | | times for the remaining tasks in that path will each be |
| 3. Are the task and the timing of the task completion | | | | reduced by that amount.) |
| critical to the organization? The effort involved in | | | | Note: A dummy is an extra node symbol used to |
| creating a diagram should be applied to tasks that | | | | clarify an activity network diagram if one node has |
| have little margin for timing error and have either | | | | more than one job or task feeding into it from one |
| serious consequences if completion of the overall task | | | | other node. Since the diagram cannot show two |
| is delayed or large rewards if completion can be sped | | | | activities coming from one node and going to another, |
| up. | | | | a dummy node is created, with zero as the job |
| Critical path mapping can be especially valuable in | | | | duration shown on the arrow connecting them. |
| project bounding and in the measure and improve | | | | Review the completed activity network diagram with |
| phases of the Lean Six Sigma methodology. | | | | the people who will be doing the work described by it. |
| How do you do it? | | | | Consider any feedback. Expand or modify the diagram |
| 1. Assemble the right team. The team must have either | | | | as necessary to fit the actual situation. |
| personal knowledge of the timing of all the subtasks | | | | Now what? |
| involved or connection to that information. The team | | | | As a tool, the activity network diagram is like a |
| should include managers and other employees as | | | | time-map of any time-sensitive project. As you are |
| close to the actual situation as possible. | | | | proceeding down the paths of the project, the map will |
| 2. Identify all of the subtasks necessary to complete | | | | help keep you on track. If you should falter or wander |
| the overall task. You might use brainstorming | | | | off the path, the map can be used to help get you |
| techniques or begin with a list of tasks from a previous | | | | back on the critical path. This description of the activity |
| project. Record the tasks so that you can rearrange | | | | network diagram will help you to manually calculate |
| them. A good way to do this is to write each job on | | | | and construct a process map. |
| the top half of a 3x5 card or Post-It(TM) note. (The | | | | Automated tools, such as SigmaFlow, are also |
| bottom half of the card will be used for timing data | | | | available. SigmaFlow produces other valuable |
| later in the process.) | | | | scheduling information, as well. Computer scheduling |
| 3. Put the activity cards in the sequence in which they | | | | programs like SigmaFlow are able to easily deal with |
| must be performed to complete the overall task. To | | | | complex processes, recalculating times whenever you |
| do this, create paths or strings of tasks that follow one | | | | modify the data. |
| after the other. These strings will often describe | | | | A critical path map can help uncover opportunities for |
| sequences of activities that occur in parallel with each | | | | increasing speed. Used in this fashion, critical path |
| other. After all the activities are in some string or path | | | | mapping is another tool to make the Lean Six Sigma |
| of activities, create the overall sequence by connecting | | | | method work smoothly. It is important to remember, |
| the paths. These connections will show where jobs or | | | | however, that these diagrams, whether produced |
| tasks require input from parallel sequences before the | | | | manually or by computer, are not intended to drive you, |
| next task can begin. Feel free to add new cards for | | | | but to signal you if something is wrong. |
| missing tasks or to remove duplicates. | | | | |