The drum resource must be shared across all projects you consider part of the multi-project environment. This is the definition of a multiproject environment. Larger companies may have several independent project groupings that share resources within the group but not across groups. In this case, you should have multiple drums.
Resources often appear as constraints. The company capacity constraint sometimes may seem to "float." The basic TOC makes it unlikely that there is in fact more than one constraint (unless you have an unstable system)! Statistical fluctuations can make temporary capacity constraints. For example, suppose a number of projects happen to demand a particular resource at one time, exceeding the resource capability. This is a statistical occurrence. We should expect it to happen. It does not make this resource a company-capacity constraint. It does mean our project plan and control system have to handle it, even if only through the individual buffers we have already added. There is also some flexibility in resource supply. On occasion, we can use overtime or ask people to defer time off. We can segment the work to ensure that we are properly exploiting the potential constraint. We can subordinate other work that does not produce immediate throughput.
However, many companies will have a chronic resource constraint. The department is always on overtime or always seems to run late. It has presumably been permitted to occupy this position because of some policy or for another reason that prohibits providing enough of the resource to meet all demands. If two or more resources seem to contend for this honor, pick the resource demanded nearest the beginning of projects. That leaves you the option to change your mind later if necessary. We can call this the capacity-constraint resource because it influences overall company performance. There must be a reason that we cannot easily increase the supply of this resource. This resource is the company bottleneck and, therefore, must become the drum for all of the projects.
Since the purpose of selecting the drum resource is to stagger the start of the projects and avoid overloading the system, it usually does not matter much if you select the wrong resource as the drum. You will still get some degree of project staggering. As long as you choose a relatively highly loaded resource that you cannot easily elevate, you are likely to get a large benefit. Project performance will help you focus on the correct drum resource over time. It is far better to get on with the drum schedule with the wrong resource than to continue to operate the old way while agonizing over the actual drum resource.
Many criteria have been proposed to identify the drum resource. With project plans, you do have the total resource demand, and you should know your total resource on hand. You could select the drum by the highest ratio of demand to available staff. You should only use this method if you have some reason to believe both of these numbers for all projects. Goldratt does not recommend this method for production because he claims your data is never very good. This may also be true for projects. If you use this method, you should ensure that the resource selected is not easily elevated, for example by hiring contractors or temporary staff.
In order to achieve the maximum effect of staggering the projects, the drum resource should be the resource that controls the largest amount of critical-chain time on your projects. This resource may vary from project to project. If, like many companies, your projects tend to follow a repetitive pattern (e.g., from engineering to construction to operation), you may find one resource that dominates critical-chain time. Selecting this resource makes it most likely that you will remove resource contention for all of the other resources in the project.
Avoid Assigning Resources by Individual Name Many companies choose to identify resources by individual names. They feel that the resources are so highly specialized that they cannot do otherwise. If this is true, you have no other option. I will observe, however, that your company is at high risk if your total multiproject throughput is controlled by one or more individuals who, if they leave or get sick, will bring all of your projects to a halt. You should consider this situation as part of your project-risk-management approach.
The preferred approach is to assign resources by type in your plans and then have the resource manager assign specific individuals as the task comes up to be performed. The definition of a resource type, then, must be that any person with that designation could do the tasks assigned to that resource type. The primary advantage to assigning resources by type is that the larger the resource pool, the more opportunity you have to dynamically assign resources to projects as the activities demand. This applies to all resources, not just the drum resource. You may, when the task allows it, further accelerate tasks by assigning more than one resource of the type to the task.
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What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.