College of Engineering and Applied Science University of Wisconsin -- Milwaukee

Transcription

1 Edward A. Beimborn and W. Anthony Garvey College of Engineering and Applied Science University of Wisconsin -- Milwaukee

2 The Blob Chart:: A Simplified Method of Allocating Resources over Time Edward Beimborn and W. Anthony Garvey College of Engineering and Applied Science University of Wisconsin-Milwaukee INTRODUCTION The problem of efficiently allocating resources over time to meet an objective is one often faced by anyone concerned in the planning of large-scale projects. Generally such a planner proceeds by defining the tasks that need to be performed, assessing the level of resources that will be available and allocating those resources over time to the various tasks involved. They may develop task flow diagrams, Gantt charts or utilize PERT (Program Evaluation and Review Technique) to assist in the planning effort. The blob chart is an additional aid which complements these other techniques and can serve as a graphical means of representing resource allocations over time. This article will describe the nature of the blob chart, and some of its features, the process that can be used to develop such a chart, and how such a chart might be converted to a Gantt chart or a PERT network. The blob chart was originally developed to aid in the planning and coordination of student group, semester long projects at the College of Engineering and Applied Science at the University of Wisconsin -- Milwaukee. The technique has been successful in helping students meet a deadline with a complex coordinated effort and at the same time give them some appreciation of the needs and demands of resource allocation problems. Because of the appeal of this procedure it was felt desirable to share it with others who may be concerned with similar problems. -1- D:\Courses\CE790\BlobChart.doc

3 Description of a Blob Chart The blob chart consists of a two dimensional diagram with one dimension being the time span over which the project will operate and the other dimension the resources available. Such a chart is shown in Figure 1. Blobs have been fitted into this chart representing tasks to be performed in the project and their time, scheduling and resource requirements. These blobs are then reshaped to fit within the constraints of time and resources associated with the project. As one might expect from the definition of blobs as tasks, their fundamental property is that they obey Parkinsons Law. That is, blobs expand and distort to consume available resources and time. This property, however, can finally be put to some advantage as will be shown shortly. In using a blob chart, there are three constraints which limit or define the final chart. The first is the time dimension. A project may have a firm time line that must be followed, or it may only have an upper bound on completion, with no essential intermediate points in time that are of interest. In the latter case, for example, one might be seeking the earliest possible completion time. The second constraint is resources. These may or may not be fixed in kind or in number. For example one may be interested in the earliest completion date with only an upper bound on the number and kind of resources, or with no limit at all. Clearly the interplay of Parkinsons Law and the Law of Diminishing Returns will both be involved. The last major constraint that must be identified is the precedence relationships between tasks, familiar from the use of CPM and PERT. -2- D:\Courses\CE790\BlobChart.doc

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5 A blob chart that was developed in the course of a student project at the University of Wisconsin-Milwaukee is shown in Figure 2. This project and its blob chart will be used to illustrate the remainder of this paper. The purpose of the project was to examine the transportation problems of the elderly and to find an optimal solution to the problem of increasing the mobility of the elderly in a socioeconomically and politically feasible manner. The students developed sufficient background material through the literature and interviews with the elderly and relevant social agency staff to develop and screen an extensive set of alternative solutions. A cost-effectiveness model and a set of developed criteria were then used to analyze the promising alternatives. Financial and social feasibility were then used to establish the preferred solution. All of this culminated in a single spaced report of fifty-five pages submitted at the end of one semester. Features: It is possible to note milestones, periods of slack, critical paths and the interrelationships of tasks in the blob chart (see Figure 3). Milestones can be noted as horizontal lines on this diagram. Slack appears as blank spaces. The sequence of tasks that occur with no slack associated with them form a critical path through the diagram. Finally the interrelationship of tasks can also be represented on a blob chart by looking at their general position on the diagram in relation to the other tasks. With these features, blob charts have proven useful in the overall planning and operation of projects. The following section will discuss the process whereby a blob chart can be developed. -4- D:\Courses\CE790\BlobChart.doc

8 DEVELOPMENT OF A BLOB CHART The development of a blob chart is a relatively simple task. It can be observed that most tasks follow a general time and resource pattern as shown in Figure 4. A task generally begins with a small level of effort on the part of a few people mainly aimed at conceptualization and planning of the task itself. As time goes on the number of people involved in the project will be expanded and the blob reaches its full width. Finally when the task is near completion, the number of persons involved decreases as the final details of the task tapers off. The development of a blob chart can follow the following sequence of steps: 1) Identify the boundaries of the overall chart, e.g. determine the amount of time that will be available for the project and the level of resources that might be available at different points in time over the length of the project. 2) Identify the tasks that need to be performed: the tasks that should be performed in a project should be defined and their sequence should be determined. A task flow diagram may be a useful way of developing this step (Figure 5). 3) Determine blobs for each task (Figure 6). The resource and time requirements for each task should next be determined. This can be best represented in terms of a blob such as shown earlier in Figure 4. 4) Fit blobs into the blob chart boundaries: Given knowledge of the constraints on the project, and the resource and sequencing needs of the tasks involved, the blob chart itself can be formed. This is done by fitting the blobs into the constraints and adjusting their shape as necessary and feasible to fit within the diagram. At this point it is possible to see if the overall resources needs and time available for the project are reasonable. If they are not it will be difficult to fit the blobs in or large amount of slack will appear. -7- D:\Courses\CE790\BlobChart.doc

11 For example, the tentative blobs of figure 6 are fitted together as a blob chart format in figure 7. In this instance the plan is ambitious in both resources and time, while at the same time giving evidence of slack. Figure 2 shows the adjusted blob chart that meets the constraints on time and resources. The number of iterations required to produce an acceptable blob chart varies and depends upon how well the tasks are known as well as the resources required. In figure 2 there is still slack but further changes could, in many instances, be inadvisable from a planning standpoint. The limitations on continued adjustment of the blobs to fill the chart is usually due to the impreciseness of task definition and awareness of resource needs as well as a desire to account for their inherent expansion. The conversion of the blob chart to a Gantt chart is straightforward. The duration and time of initiation and completion for each task can be read directly off the blob chart and inserted in a Gantt chart. Conversion of a blob chart to a PERT diagram is done in a similar fashion. Expected activity times are read directly from the blob chart and are amended with optimistic and pessimistic estimates or probabilities. In terms of the precedence relationships that must be adhered to in a PERT or CPM chart, the blob chart would seem to present some ambiguities. That is, there is extensive overlap of tasks in the blob chart making the necessary distinction of events in the PERT method unclear. But, as was alluded to in figure 6, blobs often consist of a number of subtasks, which together with appropriate resource needs yield the resultant blob shape. Utilizing this to introduce "dummy" events and activities (that have real time requirements) will alleviate the problem. For the remaining events, the precedence relationships between tasks are clear from the blob chart D:\Courses\CE790\BlobChart.doc

13 CONCLUSION The above procedure can be modified to meet the needs of a particular project. It may occur that the resources are fixed and that the total amount of time should be minimized. In this case the lower boundary of the diagram would be as high as the diagram as possible. Another variation would be to have the time span fixed and the amount of resources variable. In such a case, the right boundary would be pushed as far left as possible. This type of a chart would indicate the level of resources needed at each point in time and points where overtime or excess help might be needed. Finally it should be pointed out that the development of such a chart is an iterative process with a number of versions developed prior to the final chart D:\Courses\CE790\BlobChart.doc