Except for date constraints, the order to execute activities is usually dictated by one depending on the other — a logical relationship. In Merlin Project, dependencies are established by linking activities (and/or milestones) together.
There are three categories of relationship between activities:
- Logical: You can’t paint a wall before it’s built.
- Resource-based: This occurs when an activity could be finished sooner if you had more resources.
- Preference: The Project Manager chooses to schedule in this particular order.
And there are four standard types of dependencies:
- Finish to start: Means activity B can’t start before A is finished.
- Finish to finish: Means A must be complete before B can finish.
- Start to start: Means B can start after A has started.
- Start to finish: Means B can’t finish before A starts. (rarely used)
Dependencies establish the links and the type of links between all the activities of a project. After you have prepared your Work Breakdown Structure, you establish the dependencies to begin identifying the critical path of your project.
In Merlin Project, you’ll link activities together in Work Breakdown view by clicking on the Toolbar > Link icon and selecting the option you need. By shift-clicking (consecutive selection) or command-clicking (non-consecutive selection), you can link together multiple activities. The activities will automatically shift in time if that is what the type of dependency leads to.
You can enter a lead or lag time for activities that depend on each other. The definition for lag is “the amount of time whereby a successor activity will be delayed with respect to a predecessor activity”. Lead is “the amount of time whereby a successor activity can be advanced with respect to a predecessor activity” (See Wikipedia on Dependency in the context of project management). In Merlin Project, lead and lag time can be entered by selecting one activity, then clicking the Link icon in the Inspector and entering a lead and/or lag time in the Predecessor/Successor fields.
Another way is to click on the links themselves to reveal additional options. Having selected a link, the Inspector will allow you to go to the predecessor/successor, change type and lead/lag and show you information about the critical path. The Expected Critical option contains information on the Critical Path of the dependency in relation to the expected values. The Planned Critical option contains information on the Critical Path of the dependency in relation to the planned values.
Merlin Project supports the Critical Path Method (CPM). It requires the project to include a list of all activities, the time that each activity will take to complete, the dependencies between the activities and logical end points such as milestones or deliverables.
From these values, Merlin Project calculates the longest path of planned activities to logical end points or to the end of the project, and the earliest and latest that each activity can start and finish without making the project longer. This so-called critical path is the sequence of activities which add up to the longest overall duration. This determines the shortest time possible to complete the project.
In Merlin Project you can view the critical path simply by clicking the Toolbar > Critical Path icon or by going to View > Filters > On Critical Path. The latter will show you only the activities that are on the critical path while the former will just colour them red. Any change in a linked activity on the critical path will automatically lead to a deadline change for the project.