The economic design and application of information system of X-bar control charts under a preventive maintenance policy for multiple assignable causes

In this thesis, we assumed that the occurrences of assignable causes are according to general distribution and its failure rate is increasing. After sampling, we assumed that if the process is in control, a preventive maintenance program could be implemented immediately; otherwise, if the system is out of control, we have to reset the system to in-control state. There are multiple of assignable causes. We derive an economic design of (X) over bar control chart, the objective is to determine the design parameters that minimize the long term expected cost per unit time. In the last, the numerical example is proposed to obtain the optimal sampling sizes, sampling intervals, and control limits. In the results, we find that if the increasing failure rate of the system is faster, the cost of saving is larger. When the cost of preventive maintenance is increasing, the average cost per unit time, sampling size, sampling intervals are also increasing, and the control limits is decreasing. The results can be done as the reference of industry for the design of control chart. We also use Visual Basic to develop a software of application, left as useful tool of designing a control chart.