We now look at three more basic tools for planning.
* Gantt Charts
* Analysis System
* program-me evaluation and review techniques(PERT).
All these tools can be used in forward or reverse mode, i.e. they can be used to schedule forward from a given date to find the overall completion time or back from a given end time to determine the necessary start time. they are appropriate. and useful, techniques for production planning. in the areas of jobbing or batch production, the tools can be limited in the degree of complexity that can be handled. computer based. in any case, computer based tools can reduce the 'visibility' of what is happening operationally an important element to ensure that the manager still feels in control.
it should be noted that the aim of this module is to provide an appreciation of the strengths and weaknesses of these techniques rather than a complete background.
GANTT CHARTS
at the start of the century an objective planning method for projects was developed by henry gant. in its basic from (figure 1.1), the horizontal divisions represent time and the vertical rows item's to be scheduled. lines, bars, brackets, shading and other devices can mark the start, duration and end of a schedule. the purpose of a gantt chart is to clarify and thus improve understanding and serve as a focus for discussion.
In manufacturing,charts are used in the following situations:
* the number of work-centers is low . its use is precluded where the numbers are large due to queuing effects and the need to keep the chart dynamic. this,in itself,becomes too big a task to make it worthwhile.
* job duration is measured in days and weeks rather than hours.
otherwise again, it becomes too difficult to reflect the dynamics of the system.
* tugs are short. otherwise the effects of queuing will add a random element into each stage, making the chart unworkable.
there are a large number of reservations with the gant technique. most concern its ability to provide a control function as the complexity of the work increases. it cannot account for hold-ups, for plans only every being 'best estimates' or for plans being undermined by murphy's law (anything that can go wrong, will go wrong).
Schedule for jobs M T W Th F S Su
Wages * *
Purchases * * *
Revenue * * *
FIGURE 1.1: A simple Grantt chart-an example chart for an accounts clerk.
The major shortcoming of a gantt chart is that does not provide a method of determine how resources may be optimally allocated. for example, if manpower could be shifted from one activity to another, the second activity might be completed in a shorter time with the first taking longer. would this might be completed in a shorter time with the first taking longer. would this benefit the company overall?this kind of 'what-if?' scenario is not feasible under gantt. also, there is no correlation between activities and cost.
PROGRAM-ME EVALUATION AND REVIEW TECHNIQUES (PERT)
PERT was developed in 1958 by booz allen & hamilton Inc. under contract to the US navy special prijects division for use in the polaris project. It aims to rectify some of the weaknesses of CPA.
CPA only considers single point values for the start and finish times of activities. but it is unlikely that the 'crystal-ball gazing' that takes place in there will be no errors. to take account of the inherent variability in activity time, a distribution is used to characterize the probability of activities finishing within a given margin before/after the estimated duration time.
the following steps are required in developing a PERT analysis. the first steps follow the same procedure needed to carry out a PERT analysis. The first steps follow the same procedure needed to carry out a critical path analysis.
* earliest start time (EST) for the process or project.
* earliest finish time (EFT) for the process or Project.
The earliest finish time (EFT) is given by the earliest start time (EST), plus
the overall activity duration (DUR) encompassing all activities.
No comments:
Post a Comment