Identification of these activities makes it easier for management to identify which areas require more attention and resources on a priority basis and which ones can be put on hold. In this blog post, we take a brief look at why critical activities matter and how they should be handled by project managers. The critical path outlines the longest distance between the start and the end of a project.
It includes all the tasks, resources, and activities that must be completed to finish the project on time. The critical path also forms the basis of the project schedule and indicates the time it would take to complete the work. There are a few terms used in critical path modeling that make it easier to understand how this model works. This is the earliest date at which an activity can begin. Most projects have many activities that take place successively, one after the other.
An activity that is dependent on the completion of a previous task is generally given the earliest start day based on the time it would take to complete the preceding activity. The earliest finish date is the minimum amount of time it would take to complete an activity. In most cases, it is understood that the activity must receive full attention and resources from the project in order to meet its earliest finish date.
The latest start date is the latest date by which an activity can start due to delays in prior tasks. Latest start dates are particularly important for earlier activities on a project because later activities are dependent on their timely completion. This is the latest finish date for a particular project activity.
Delays in the completion of activities, particularly the earlier activities, can push the project off-schedule significantly and make it difficult to complete on time. Every activity on the project has a float that determines how long that task can be delayed before it has an impact on the project schedule. The float is calculated by subtracting the earliest start date from the latest start date or the latest finish date from the earliest finish date on the project.
Every project activity has a crash period which is the shortest amount of time required to complete the task. How are critical activities identified? Float is a measure of schedule flexibility. If the Early Start date and Late Start date for an activity are the same, the activity is said to have zero float. Activities that have zero float must start on time to prevent the schedule from slipping. Any delay in an activity on the critical path would reduce the amount of total float available on the project.
A project can also have negative float , which means the calculated completion date of the last activity is later than the targeted completion date established at the beginning of the project. If activities that are not on the critical path have a difference between their early start date and their late start date, those activities can be delayed without affecting the project completion date.
The float on those activities is called free float. The next step is to work through the network diagram from right to left beginning with the mandated completion date, which is a milestone that is set in the project plan. Subtract the duration of each activity in each path to determine the latest date the activity could begin and still meet the project completion date. Resource calendars must be considered in the backward pass as well as the forward pass.
To calculate late start dates, begin with the project completion milestone and assign that date as the finish date of its predecessor activities. Follow these steps to calculate the late start dates of predecessor activities, assuming finish-start relationships:.
The difference between the early start date and the late start date for activities on the critical path is usually the same as the total float, unless the activities are affected by the resource calendars differently in the forward and backward pass.
For example, if a piece of key equipment is only available for a few days, activities that depend on it have the same start and finish dates in the forward and backward passes. Project Management for Instructional Designers by Wiley, et al.
The list of prerequisites for a job may include more than the immediate prerequisites; e. Job Times. An accurate estimate of total project time depends, of course, on accurate job-time data. CPM eliminates the necessity and expense of careful time studies for all jobs. Instead the following procedure can be used:. In many projects studied, it has been found that only a small fraction of jobs are critical; so it is likely that refined time studies will be needed for relatively few jobs in a project in order to arrive at a reasonably accurate estimate of the total project time.
CPM thus can be used to reduce the problem of Type I errors at a small total cost. A computer algorithm has been developed to check for errors of Types 2 and 3 above.
The algorithm mentioned in footnote 4 systematically examines the set of prerequisites for each job and cancels from the set all but immediate predecessor jobs. Wrong or Missing Facts. Errors of Types 4 and 5 cannot be discovered by computer routines. Instead, manual checking perhaps by a committee is necessary to see that prerequisites are accurately reported.
The cost of carrying out a project can be readily calculated from the job data if the cost of doing each job is included in the data. If jobs are done by crews, and the speed with which the job is done depends on the crew size, then it is possible to shorten or lengthen the project time by adding or removing men from crews.
Other means for compressing job times might also be found; but any speedup is likely to carry a price tag. If we want to shorten the project, we can assign some of the critical jobs to their crash time, and compute the corresponding direct cost. In this way it is possible to calculate the cost of completing the project in various total times, with the direct costs increasing as the over-all time decreases.
Added to direct costs are certain overhead expenses which are usually allocated on the basis of total project time. Fixed costs per project thus decrease as project time is shortened.
In ordinary circumstances a combination of fixed and direct costs as a function of total project time would probably fall into the pattern shown in Exhibit VII. The minimum total cost point A would likely fall to the left of the minimum point on the direct cost curve point B indicating that the optimum project time is somewhat shorter than an analysis of direct costs only would indicate.
Other economic factors, of course, can be included in the analysis. For example, pricing might be brought in:. A large chemical company starts to build a plant for producing a new chemical. After the construction schedule and completion date are established, an important potential customer indicates a willingness to pay a premium price for the new chemical if it can be made available earlier than scheduled. With a plot of costs correlated with total project time, the producer is able to select a new completion date such that the increased costs are met by the additional revenue offered by the customer.
This effort is sparked, in part, because of the Air Force and other governmental agency requirements that contractors use these methods in planning and monitoring their work. Here are some illustrations of progress made:. One of the present authors Wiest has developed extensions of the work-load smoothing algorithm.
Indications of how smoothing can be accomplished are also made. For the manager of large projects, CPM is a powerful and flexible tool, indeed, for decision making:. Because of the above characteristics of CPM—and especially its intuitive logic and graphic appeal—it is a decision-making tool which can find wide appreciation at all levels of management.
For the manager concerned with day-to-day operations in all departments, CPM enables him to measure progress or lack of it against plans and to take appropriate action quickly when needed. And the underlying simplicity of CPM and its ability to focus attention on crucial problem areas of large projects make it an ideal tool for the top manager.
On his shoulders falls the ultimate responsibility for over-all planning and coordination of such projects in the light of company-wide objectives. For a method for smoothing operations in a job shop, based on CPM and the use of slack, see F. Levy, G. Thompson, and J. An algorithm on which one such computer program is based is discussed by F.
See A. Charnes and W.
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