To find the critical path on your own, locate the task with the latest finish date, and then trace back through its predecessors until you reach the beginning of the project. Each task on the critical path is called a critical task.
To be precise, the critical path is the longest sequence of tasks without any scheduling leeway (called slack time). Slack time is the amount of time a task's finish date can move without affecting another part of the project. Free slack is how much a task can move without affecting its successors. Total slack is the time a task can move without affecting the end of the project. The lack of slack time is the reason critical tasks affect a project's finish. (If you want insurance for your project's finish date, you can add buffers to your schedule. Then, if tasks delay, they eat into the buffers instead of delaying the finish date.)
Reality Check: Double-Checking the Schedule
Mistakes will always linger in your project schedule, no matter how many times you check it. Moreover, as you refine the schedule, you can introduce mistakes that weren't there before. So as you fine-tune the schedule, stay on the lookout for errors. For example, you may notice a task scheduled when you know the assigned resource is on vacation, or spot the wrong kind of task dependency between two tasks. Watch for the following items:
- Task dependencies that shouldn't be there or should be a different type.
- Tasks with inflexible date constraints that they shouldn't have.
- Manually scheduled tasks that you should have switched to auto-scheduled.
- Work or duration values that seem too low or too high.
- Work package tasks without assigned resources.
- Summary tasks with assigned resources.
- Overallocated resources.
- Resource calendars that don't represent people's actual availability.
In addition, make sure you've included tasks and time for often-overlooked types of work, like project-related meetings (including status meetings and project management meetings) and work reviews. Because several people attend these powwows, the time and expenses add up fast. Even approvals aren't as simple as signing the sign-off sheet. Be sure to include time for people to review documents or other deliverables before they approve them.
Note: Microsoft didn't come up with the concept of the critical path. Project uses the critical path method (CPM) to calculate the start and finish dates for tasks. To learn about the critical path method and its task date calculations, check out Project Planning, Scheduling & Control, by James P. Lewis (McGraw-Hill). (Chapter 5 of that book has the CPM information.)
Driving somewhere is a simple example of a critical path and slack time. Suppose you and your brother are both heading to your elderly Aunt Thelma's for lunch, which starts at noon sharp. You're taking the highway, which takes 65 minutes. Your brother, on the other hand, is driving his sports car over the scenic route, which is 90 minutes of twists and turns. Your brother's drive is the critical path, because it takes the longest. That gives you 25 minutes of slack time before you must start driving (or you can use the time to stop and pick flowers for your dear auntie).
Once you start executing a project, you must keep close watch on the critical path. Delays that occur in critical tasks directly affect the project finish date.
Displaying the critical path in Project
The standard Gantt Chart view doesn't show the critical path or slack time initially. All you see are blue task bars. However, in Project 2010, you can easily format the view to display the critical path and slack time. Choose Format. In the Bar Styles section, turn on the Critical Tasks checkbox and the Slack checkbox. The Gantt Chart displays critical tasks in red and shows slack as narrow black bars at the right end of task bars.
The critical path appears in red in the Tracking Gantt view (choose Task→View, click the down arrow, and then choose Tracking Gantt) and the Detail Gantt view (choose Task→View, click the down arrow, choose→More Views, and then double-click Detail Gantt). The Tracking Gantt view displays gray task bars for the baseline schedule, blue task bars for noncritical tasks, and the Entry table in the table area. The Detail Gantt view shows noncritical path task bars in blue and critical path task bars in red along with the Delay table so you can evaluate leveling delays.
Filtering the task list to show critical tasks
Filtering the task list to show only critical path tasks helps you focus on shortening the tasks with the most bang for the buck. When you use filters, you must reapply them regularly, like sunscreen. To filter the task list to show only critical path tasks, do the following:
- Choose View→Data. Click the down arrow next to the filter box, and then choose Critical.
Project shows critical path tasks and the summary tasks to which they belong.
- To hide the summary tasks, choose Format→Show/Hide, and then turn off the Summary Tasks checkbox.
The summary tasks disappear, so you see only critical work package tasks.
Tip: Grouping by critical status keeps noncritical tasks visible but out of the way. Click the down arrow to the right of the Task Name column heading. In the drop-down menu, choose "Group by"→Critical. Project lists all the noncritical tasks first (under Critical: No), and then lists critical tasks (under Critical: Yes). Revert to the regular order by right-clicking the down arrow to the right of the Task Name column heading and then choosing No Group.
Showing critical tasks in a Gantt Chart table
Bright red task bars in the timescale make the critical path easy to see. But the tasks in the table area still look exactly the same. Project has formatting features that can highlight the critical path in tables as well. If you want to see the critical path even in the Gantt Chart table area, do the following:
- To change the appearance of the table text for critical tasks, choose Format→Format→Text Styles.
The Text Styles dialog box opens. The advantage to formatting text styles instead of individual text elements is that Project applies the style whenever it's appropriate. For example, if you modify the appearance of text for critical tasks, then Project applies or removes the formatting as tasks join or drop off the critical path, making it easier for you to spot the changes.
- In the Item to Change drop-down list, choose Critical Tasks.
Any changes you make to the font or colors apply to all critical path tasks.
- Choose the font, font style, and font size.
The Font list displays the fonts installed on your computer. The "Font style" list controls whether or not the text is bold or in italics. The Size list includes the standard font sizes. (You can type a number in the box to specify a font size not in the list.) Turn on the Underline checkbox if you want the text underlined as well.
- Optionally, to change the font color, in the Color drop-down list, choose the color you want (for example, red to match the task bars).
Colors other than black can make text hard to read. If you do change text color, opt for dark colors. In addition, remember that colors may not reproduce well when you print in black and white. Consider yourself warned.
- To highlight cells that use a text style, in the Background Color drop-down list, choose the color you want.
For example, you can change the background for critical cells to red (shown as light gray highlighting in Figure 12-2). As you select options and settings, the Sample box previews the text appearance.
- Click OK.
Cells that use the text style immediately show the new formatting. The background color and pattern apply to all critical task cells, except for the Indicators column. These changes appear every time you apply the current table. You can also create a custom table to show critical tasks in this way.
Learn more about this topic from Microsoft Project 2010: The Missing Manual.
Microsoft Project 2010 helps users control the variables on any project, big or small -- such as schedules, budgets, communications, and changes -- rather than be controlled by them. Written by project management expert Bonnie Biafore, this book teaches you how to do everything from setting, tracking, and adjusting schedules and budgets to testing scenarios and recognizing trouble spots before your project breaks down.