Friday 23 April 2021

How to schedule a project

This article discusses the methods of scheduling the project, analyzed them, listed the features of the methods, their merits and disadvantages. Recommendations for how to use project management techniques, depending on the type and specifics of the project, are provided.

Project management is an application of knowledge, skills, tools, and techniques to work on a project to meet the project requirements. The result of effective management is the implementation of the project within the necessary time frame, budget and in accordance with the initial requirements. 

A key factor in the success of project management is the availability of a clear per-determined project schedule.

Scheduling is the process of analyzing operations sequences, duration, resource requirements, and time constraints to create a project schedule. Putting operations, duration, and resources into a scheduling tool generates a timetable with scheduled project completion dates. 


Developing an acceptable project schedule is often an iterative process, i.e. specific stages of scheduling take place multiple times. As a result, the planned start and finish dates of operations and control events of the project are determined. Managing the project schedule includes all planning, monitoring and schedule adjustment activities. 


t is the project schedule that allows you to use resources in the most efficient way, to attract them to the dates when they are needed and to release when the need for resources is no longer necessary. From a financial point of view, the company will be able to plan its payments through the project schedule, which avoids such negative things as cash gaps. 

The revision of the schedule and its realism continues throughout the project as the work progresses, changes the project management plan, and identifies the nature of the risk events. Below is a timeline chart for the project.



The following basic methods of scheduling a project are used when managing a project:

  1. PERT method.
  2. Critical path method.
  3. The critical chain method.

How to schedule a project

PERT method

PERT was established in the late 1950s by the U.S. Navy to accelerate the development of the Polaris submarine ballistic missile. Several thousand private contractors and government organizations had to be coordinated in the development of this weapons system. 


Coordination of work was so successful that the entire project was completed two years ahead of schedule. This has led to the continued use of PERT in other weapons development programs in the U.S. Navy, Air Force and Land Wax. It is now widely used in industry as well as in service organizations.

Usually, the time needed to carry out various works is unknown in advance when conducting research and development. Therefore, PERT takes into account the uncertainty in the job duration. The method allows you to determine the probability of completing different stages of the project in a given time, as well as calculate the expected duration of the project. 

An important and extremely useful result of PERT is to identify project bottlenecks. In other words, work that is more likely to delay the completion of a project is identified. Thus, even before the work begins, the project manager knows where delays can be expected. They have the capacity to take the necessary measures in advance to eliminate possible delays and to ensure that the project is implemented on time.

There are a number of features in the PERT method:

  1. PERT should only be used for large projects with more than 300 jobs. In addition to a sufficient number of critical path works, this will ensure the independence of the random quantities of their duration.
  2. To apply PERT, you need to select experts and organize their work to get estimates of optimistic, pessimistic and the most likely duration for each project work. The quality of this work will depend on the quality of PERT
  3. PERT understates the estimate of the duration of the project. The more parallel works, the more serious the error. Monte Carlo should be used to get it done.
  4. A critical way for a project to implement it may be a different path from the one obtained by the PERT method. The degree of criticality of a project also depends on a specific implementation. We can only talk about the probability that the work will be critical.


PERT does not take into account existing constraints on the resources and actions of the project manager, who seeks to complete the project within the appointed time frame. To be successful, PERT requires one assumption: all random times of the duration of critical path work are independent. Otherwise, this will affect the duration of the project.

The main difference between PERT and CPM is that the duration of the work is considered random values. In other words, PERT allows you to take into account the uncertainty of the actual duration of the project's work to assess and analyze the timing of its execution.

The PERT method takes a probabilistic approach to determining the duration of work using the average PERT β distribution is widely used in research and development projects, as it allows to take into account the uncertainty of the timing of the work. 

Critical Path Method

The Critical Path Method (ICP) is in many ways reminiscent of PERT, but was developed independently by Dupont de Nemur. In fact, both PERT and CPM methods are developed almost simultaneously. The main difference between the two is that CPM does not take into account random variations in the duration of work.

Instead, it is assumed that the duration of work is proportional to the amount of resources allocated and that by changing the amount of resources, the length of work and the timing of the project's completion can be changed. Thus, based on the experience of similar projects, the ICP establishes a relationship between available resources and duration of work. The cost-to-trade ratios and the duration of the project are then assessed.

The ICP makes the following requirements for the model of the project.

  • The project consists of a certain set of works. All work in the process of the project must be completed and no other work can arise.
  • Each work knows how long it will take place.
  • A pre-existing attitude has been introduced on many works. The beginning of each subsequent work is influenced only by the end of previous works and the relationship of the previous work.

The critical path method allows you to calculate the theoretical dates of the early start and finish, as well as the dates of late start and finish for all operations without taking into account resource constraints by analyzing the passage back and forth across the project network:

  • Passage forward. The earliest possible deadlines for the project are calculated.
  • Passage backwards. The latest possible deadlines for the project are calculated.

The resulting early start and finish dates are not always the project schedule; they indicate the time periods in which operations can be scheduled, taking into account the duration of operations, logical connections, ahead of time, delays and other known limitations.

The calculated early and late start and finish dates may be affected by the overall operating time reserve, which is defined as the difference between late and early deadlines. This allows the schedule to be flexible and can be positive, negative or zero. For any online journey, schedule flexibility, called "full time reserve," is measured by the positive difference between early and late dates. Critical pathways have a full time reserve, either zero or no, and planned critical operations are called "critical operations." 

A critical path is usually characterized by zero full time reserve, i.e. the longest path in the network. There may be several paths in the networks that are close to critical. Creating paths in a network with zero or positive full time reserve may require adaptation of operations duration, logical connections, ahead of time, delays, and other time constraints. 

Once the full time reserve of the journey in the network is calculated, a free time reserve can also be determined, a period of time for which the operation can be delayed without delaying the early start of any immediate follow-up operation in the network path.

Here are some steps to be taken to calculate the critical path:

  • List all tasks (operations) with the job number or number in the IMR.
  • List dependencies for each operation.
  • Write down the duration of each operation.
  • Calculate the early start date and the early end date of each job. Accordingly, when calculating early dates, you should take into account all the dependencies that this operation has.
  • Calculate the late start date and late end for each operation, taking into account the existing relationship between operations.
  • Calculate the time reserve for each operation as the difference between early and late start (or end dates). The time reserve is always a plus.
  • Identify the critical path of the project by summing up the duration of each zero-time operation.


The methodology offered by the SRM is now widespread, but it has its drawbacks. SRM optimizations lend themselves only to relatively easy-to-understand projects in which it is not difficult to predict the time of execution. Therefore, when developing or constructing different systems (when one of the intellectual tasks may not be solved for a long time) SRM is only conditionally applicable.

As a result, the ICP cannot take into account the limitations on resources, does not take into account the uncertainty of the work, does not take into account the possible risks of the project, the quality of performance.

According to numerous Studies by Standish Group for traditional project management methods, only 44% of projects usually complete on time. On average, projects occupy 222% of the originally planned duration, 189% of the initial budget. 70% of projects reduce the initial volume of work of the project, 30% of projects are closed ahead of schedule.

This method of project scheduling is used on projects where there is no tight time frame and the project can be performed for both internal needs and external clients. Violation of them is likely to entail a temporary shift in the delivery of the project and sanctions against the project manager and appropriate penalties.

Critical chain method

The method was first described in 1997 in Goldratt's book "The Critical Chain", the method met with broad support of specialists, as it was close in technology to the classical method PERT (resource connections were actually an extension of the network model to resources), and the calculated algorithms were quite simple and effective in quick action. The calculation of buffers was also simple and similar to the method of calculating the duration of work in some extensions of the PERT method and corresponded to the established practice in many organizations.

There is one very major problem in project management, with a significant number of projects running out of time, which has a negative impact on the success of these projects. Rapid implementation of projects is extremely important and directly related to profit, so companies that are able to implement their projects in a short time have a serious competitive advantage over those that regularly break deadlines. That is why Goldratt chose the main goal of the project in the minimum possible time.

By acting according to the TOC,we will identify problem project management zones under CPM/PERT that directly affect the speed of the project. The result of this analysis is the following list of problems:

  • Winning on time is not transferred.
  • The work takes all the time allotted to it.
  • Performers do not transfer work to the next stage earlier in early execution.
  • It is not possible to accurately estimate the length of each work.
  • In cases of disruption of the deadlines, corrective actions are taken related to the increase in the budget or the reduction of the volume of work.
  • Other projects are busy with the necessary resources.
  • The non-critical work required to complete the task is not yet complete.


After a detailed analysis of each of the problems, Goldratt offered his approach to problem solving:

  • Use time estimates with 50% uncertainty overlap.
  • Performers are protected from management's pressure on the timing of the work. Ensure the hard work of the performers on their task.
  • Focus on the end date of the project, not on determining the duration of each assignment.
  • The introduction of a project buffer is a total time reserve for the project to compensate for uncertainty.
  • The introduction of resource buffers is an alert to the resources of those employed on the critical chain that they will soon need to switch to a project assignment.
  • The introduction of feeding buffers is a temporary reserve to cover uncertainty when performing non-critical circuit work.
  • Thus, Goldratt laid the foundation for the formation of a new method of project schedule management, which were further developed by his followers.

A critical chain is a network analysis method that changes a project schedule to reflect limited resources. Initially, the project's network chart is based on duration estimates, dependencies, and limitations. Then the critical path is calculated. Once a critical path has been identified, resources are taken into account and the result is a timetable based on resource constraints. The resulting schedule often has a modified critical path.

A critical path with resource constraints is known as a "critical chain." The critical chain method adds duration buffers in the form of non-work operations to manage uncertainty. One of the buffers at the end of the critical chain is known as the project buffer and protects the status date of completion from delays on the critical chain. 


Additional buffers, known as "feeding buffers," are located at each point, where the critical chain includes chains of interconnected operations from outside the critical chain. Feeding buffers thus protect the critical chain from falling behind on incoming chains.


The size of each buffer should take into account the uncertainty of the duration of the chain of dependent operations leading to this buffer. Once the schedule buffer operations are defined, schedule operations are scheduled for the most late scheduled start and finish dates. 


Thus, instead of managing the full time pool of network paths, the critical chain method concentrates on managing the remaining buffer durations, comparing them with the remaining duration of the chain of operations.

Let's describe the main features of the critical chain method.c.

  • The method is aimed at the project as soon as possible.
  • Allows you to manage the project schedule in a limited environment of renewable resources.
  • It takes into account the uncertainty of the duration of work.
  • It allows to mobilize the project team to achieve its goals, having organizational and psychological mechanisms of stimulation.
  • There is a problem with assessing feed and design buffers.


The critical chain method should be used on projects where the deadline is set and the project is performed for an external customer. As a rule, such projects are completed on time, although it is necessary to adjust to the current situation speed and volume of work.

An example is the construction of facilities for the Olympic Games. No one will be able to move the Olympic Games, if the object is not surrendered, so whatever the timing, the object should be completed. In projects of this type the budget changes, but the deadlines remain constant.

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