The critical path, differences between planned, scheduled, and actual dates, lead time and lag time, and deadline dates can all affect the way Microsoft Project schedules.
As you build your schedule by entering tasks and durations, linking tasks, and setting constraints, a series of tasks that affect the project's finish date begins to emerge. If any of these critical tasks' finish dates slip, the project's finish date will slip. The word "critical" doesn't imply how important a task is; a task is critical because it must occur as scheduled for the project to finish on time. Critical tasks have no total slack. All the critical tasks together are known as the critical path. After a critical task is completed, it changes to noncritical because it can no longer affect the completion of future tasks.
By default, Microsoft Project calculates only one critical path. However, you can calculate critical paths for an independent network of critical tasks, so that you can see more than one critical path in the project. Viewing multiple critical paths can be useful for analyzing different phases or milestones or for examining the structure of subprojects inserted in your project.
You can adjust the criteria that define a critical task by specifying how little slack it can have before being considered critical. You can set this value in the Tasks are critical if slack is less than or equal to box on the Calculation tab of the Options dialog box (Tools menu).
If you need to shorten the schedule, focus your attention on critical tasks. If you have tasks that can be scheduled at the same time, you can shorten the critical path most by changing the task dependency. For example, if two tasks can begin at the same time, you can change the task dependency to start-to-start. And, if you have additional resources available, you can assign them to critical tasks and shorten the task's duration by accomplishing more work in a shorter time period.
Differences between planned, scheduled, and actual dates
After you have entered all pertinent project information and are ready to begin the project, you can set a baseline and begin to track actual progress on tasks. When you enter an actual start date or actual finish date for a task, Microsoft Project updates the scheduled dates for that task in the Start and Finish fields. The baseline start and finish dates, however, are not affected by changes you make to the actual or scheduled dates.
When you update or enter progress on tasks, Microsoft Project calculates the difference between a task's baseline start dates and the scheduled start dates, and shows that difference in the Start Variance field. The variance is also calculated between the baseline finish and scheduled finish fields, shown in the Finish Variance field. When tasks are completed, the variance is calculated between the baseline start and finish dates and the actual start and finish dates.
If changes you've made to tasks affect their start or finish dates, or their successors' start or finish dates, the new dates become scheduled start and finish dates. Scheduled start and finish dates show the current status of the project as a result of the changes you made.
You can also compare baseline, scheduled, and actual dates to dates in an interim plan if you want to see incremental progress on your project.
Lead time and lag time
Sometimes you need to model a more complex relationship than a simple task dependency. You can enter lag time to represent a delay of the successor of any task dependency. For example, for two tasks linked with a finish-to-start task dependency, you can enter lag time between the finish of the predecessor and the start of the successor task. If you have two tasks, "Paint wall" and "Hang pictures," you need a delay between the finish of "Paint wall" and the start of "Hang pictures" to allow the paint to dry. You can enter lag time as a duration, such as 1d, or as a percentage of the predecessor's duration, such as 25%. For example, if the predecessor has a four-day duration, entering 1d or 25% would result in a one-day delay between the tasks.
Lead time is an overlap of two tasks, so the successor starts before the predecessor finishes for any type of task dependency. Adding lead time can be useful if you want to give a successor task a head start. Lead time is entered as negative lag, such as -1d or -25%. For example, for the tasks linked with a finish-to-start task dependency, "Construct walls" and "Plaster walls," you can use lead time to begin "Plaster walls" when "Construct walls" is half done.
You can use any unit of duration for lead and lag time: minutes, hours, days, weeks, months, elapsed minutes, elapsed hours, elapsed days, elapsed weeks, or elapsed months.
Deadline dates don't usually affect task scheduling. Deadline dates are used to indicate a target date you don't want to miss, without requiring you to set a task constraint that could affect scheduling if predecessor tasks change. A task with a deadline is scheduled just like any other task, but when a task finishes after its deadline, Microsoft Project displays a task indicator notifying you that the task missed its deadline.
Deadline dates can affect total slack on tasks. If you enter a deadline date before the end of the task's total slack, total slack will be recalculated using the deadline date rather than the task's late finish date. The task becomes critical if the total slack reaches zero.
You can set deadlines for summary tasks as well as individual tasks. If the summary task's deadline conflicts with any of the subtasks, the deadline indicator signifies a missed deadline among the subtasks.
But, deadline dates can affect how tasks are scheduled, if you set a deadline date on a task with an As Late As Possible constraint. The task is scheduled to finish on the deadline date, though the task could still be finished beyond its deadline if predecessors slipped.