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What he meant was that there is a lot of security in a twenty-six-week task. When the start date comes, if the person doing the task is busy, she might say, "I can always make up a day on a twenty-six-week activity. I'll get started tomorrow." This procrastination may continue until she realizes she has delayed too long. Then there is a big flurry of activity as she tries to finish on time. All the work has been pushed out to the end of the twenty-six-week time frame.
A good rule of thumb to follow is that no task should have a duration much greater than four to six weeks. A twenty-six-week task can probably be broken down into five or six subtasks. This approach generally keeps people from back-end loading.
A good rule of thumb to follow is that no task should have a duration much greater than four to six weeks.
There are two ways to develop a schedule. One is to begin at the end and work back until you arrive at the beginning. The second method is to start at the beginning and work toward the end. Usually, it is easiest to start at the beginning.
The first step is to decide what can be done first. Sometimes several tasks can start at the same time. In that case, you simply draw them side-by-side and start working from there. Note the progression in the diagram in Figure 5-4. I have numbered the boxes according to the steps taken to place them. In other words, all boxes with a 1 beside them were placed in the diagram in step 1, and so on. Note that it sometimes takes several iterations before the sequencing can be worked out completely.
^ Si -
Figure 5-4 CPM diagram for yard project.
This small project might be thought of as having three phases: preparation, execution, and cleanup. There are three preparation tasks: pick up trash, put gas in equipment, and get out hedge clipper. The cleanup tasks include bagging grass, bundling clippings, and hauling trash to the dump.
Schedules should be developed according to what is logically possible; resource allocation should be done later. This yields the optimum schedule.
In doing this schedule diagram, I have followed a basic rule of scheduling—to diagram what is logically possible, then deal with resource limitations. For a yard project, if no one is helping me, then there really can be no parallel paths. On the other hand, if I can enlist help from the family or neighborhood youth, then parallel paths are possible. The rule I suggest is that you go ahead and schedule as if it were possible to get help. This is especially important to remember in a work setting, or you will never get a schedule put together. You will be worrying about who will be available to do the work and end up in analysis paralysis.
Another rule is to keep all times in the same increments. Don't mix hours and minutes—schedule everything in minutes, then convert to hours and minutes as a last step. For this schedule, I have simply kept everything in minutes.
Another rule is to keep all times in the same increments—minutes or days, for example.
I suggest that you draw your network on paper and check it for logical consistency before entering anything into a computer scheduling program. If the network has logical errors, the computer will just give you a garbage-in, garbage-out result, but it will look impressive, having come off a computer.
It is also important to remember that there is usually no single solution to a network problem. That is, someone else might draw the arrow diagram a bit differently than you have done. Parts of the diagram may have to be done in a certain order, but often there is flexibility. For example, you can't deliver papers until you have printed them, so if the diagram shows this, it is wrong. There is no single right solution, but a diagram can be said to be wrong if it violates logic.
The network for the yard project could get a lot more complicated. You could add edge front sidewalk and edge back sidewalk. You could talk about trimming around trees in both front and back. And so on. But there is no need to make it too complicated. We don't usually try to capture exactly how we will do the work, just the gist of it.
The next step is to figure out how long it will take to do the job. Time estimates for each task are made by using history—remembering how long each activity has taken in the past. Remember, though, that the estimate is valid only for the individual who is going to do the task. If my daughter, who is sixteen, does the lawn mowing using a push mower, it will probably take less time than if my son, who is only twelve, tackles the job. (In Chapter 6, we discuss how to find the critical path through the network, which helps you figure out how long the project will take.)
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