What Is the Critical Path ?
A task is critical if its finish date slipping causes the entire project to slip. The critical path is the longest connected chain of tasks that are all critical.
In the figure shown here, the red line represents the project's finish date. You can see that there are three different chains of linked tasks in this project. In the Total Slack field, the tasks that have a number greater than zero have room to move before they affect the finish date of the project. Tasks 4 “8, however, have a Total Slack of zero days. This is because Task 8's finish date and the finish date of the entire project are the same. This means that if any of the tasks in this chain slip even one hour , the whole project will finish late.
Also notice that the Total Slack value for Tasks 1 “3 is two days, even though Task 1 is four days away and Task 2 is three days away from the project's finish date. This is because the Total Slack value is really measuring the distance, in days, between the finish of the last task in the chain and the project's finish date. This is because it doesn't matter which task in the chain slips. If any of them slip, the last task in the chain moves closer to the project's finish date.
After all that, the question in your mind might still be "Who cares?" There is a whole school of thought on project scheduling that says YOU should care. The Critical Path Method ( CPM ) says that a good way to monitor your project is to watch the critical path and those tasks with very low Total Slack values because they are more likely to have an effect on the overall finish date of your project. Although the reality of project scheduling is almost always more complex than just a simple metric such as Total Slack, it is a good place to start looking. With this in mind, you can now see why working on the logical linking of your tasks is so important. These links help create logic paths through your project, and as these paths get moved around in time because of changes in schedule or 0through the reporting of actual work by resources, these paths will help you see the effect these moves will\might have on your project's finish date.