Mapping a Critical Path

A network diagram is a method of displaying the time of the various subtasks that are involved in a project. When you use it, the duration, early and late times of starting and ending each task are also counted and displayed. In addition to demonstrating which subtasks are critical to completing on time, a network diagram can help identify places where it's worth putting in the extra effort to speed up sub tasks for best results.

The network diagram has a considerable history dating back to the 1930s. In the 1950s, the method appeared as a method of planning and estimating the time spent on a project using a network diagram (PERT) and a critical path method (CPM - Critical Path Method). There are several options to provide the results of the PERT/CPM chart creation process.

This article will also look at the arrow chart method. The arrow chart works with numeric nodes as instant start/stop points for actions. The actions themselves unfold on arrows connecting the nodes.

How is this method useful?

A network diagram can show which actions or which sequence of actions are critical to the timing of a more complex set of interacting actions. This can come in handy when deciding when and where to put in the extra effort to complete the project on time. Creating a network diagram takes time, so you should consider the following points when creating it:

Is the task challenging when considering the simultaneous paths that need to be coordinated? Creating a chart for a relatively easy task can be a waste of time.

Are the durations of sub tasks more or less clearly known? If the specific duration of the action differs from the values in the chart, then it will be of little use, and it will be considered useless.


Are the actions and time to complete a task critical to the organization? The effort involved in creating the chart should be applied to tasks that have more accurate duration data and have serious consequences in the event that completion is delayed, or have a great advantage if its execution is accelerated.

A sketch of a critical path can be particularly useful in limiting the project and in the measurement and improvement phases belonging to the Lean Six Sigma methodology.

So how do you do it?

Put together a good team. The team should have either personal knowledge of the durations of all the subtasks involved, or a connection to this information. Managers and other employees should be present in the team and as close as possible to "real actions".

Identify all the subtasks that are required to complete the entire task. You can use the brainstorming method or start with a list of tasks taken from a past project. Write down the tasks so you can rebuild them. A good way to do this would be to record each action on a self-adhesive or 3x card. (The bottom of the card will be used later in the process to compile duration data.)


Arrange the action cards in the order in which they must be completed to complete the entire task. To do this, create paths or threads of tasks that follow each other. These threads will often describe sequences of actions that will arise in parallel with each other. After all the actions are included in some path or thread, create a common sequence by connecting the paths. These connections will show where tasks and work require input from parallel sequences in order to move on to the next task. You can add new cards for missing tasks or remove duplicate ones.

Assign a duration to each task or work. Write it on the bottom half of the card. Since you will be adding these duration values, you should be consistent in values. For example, you should not allow the presence of cards that do not have a completion date, or those where hours are displayed in some and minutes in others. Select the common lowest denominator.

Count the shortest duration with which a common task can be completed by adding the durations of all sub tasks, while counting their longest duration - this is the critical path. Knowing the critical path is important because it will reflect the ability to complete the task within a set time frame. The critical path determines the work that cannot be extended. Each task must be completed on time for the project to follow the plan. 

The critical path determines what is worth improving to improve the speed of execution. (If critical path tasks can be accelerated, then the total project completion time can be reduced; however, you should remember that if the task is accelerated in the critical path, then the other path may become critical.)


Calculate the earliest and latest start and finish times for each project work or subtask. Start by starting the chart. The earliest start time of each work is the total duration of all previous tasks on the road. The earliest completion time is the sum between the earliest start time and the duration of the job. Repeat this process for each task in each path. Next, calculate the latest start and finish times – start at the earliest completion time at the end of the charts.

In order to calculate the downtime of any work or task, you should subtract the earliest start time from the latest start time. All tasks in the critical path, by definition, will have 0 as the idle time. Remember that downtime depends on the completion time of the previous task. If any downtime on a path other than critical is used in an early job, the downtime in the remaining tasks will be less by a given downtime value.

 

(Note: A rogue node is used to explain a network diagram if one node has more than one task as input from the previous ones. Because a network diagram cannot display two actions originating from one node and entering the other, you use a dummy node that has 0 as the execution duration displayed on the connecting arrow.)


Review the completed network diagram with the people who will be doing the work described. Count every vote - expand or modify the chart if necessary to fit the real situation.

Now what?

As a tool, a network diagram is similar to a time map of a precision project. As you follow the path of the project, the map will help you stay on the right track. If you stumble or stray from the path, the map can be used to help you get back on the critical path. This description of the network diagram will help you manually calculate and create a process map.


Automated tools like Sigma Flow can also help you. Sigma Flow creates other valuable schedule information. Computer scheduling programs, such as Sigma Flow, can easily work with complex processes, recalculating the time for any modification of the data.


A critical path map will help you unlock the possibilities of increasing speed. When used for such purposes, the critical pathway is another tool of the method of giving flexibility in the operation of the Six Sigma technique. It is worth remembering that these charts, created manually or automatically, should not move you, but should only notify you that something is wrong.