Understanding Control Charts

Control chart is a statistical tool that shows whether a process is in control or not. Control chart is commonly used as an improvement tool.

A control chart plots a quality characteristic statistic in a time-ordered sequence. A center line indicates the process average, and two other horizontal lines called the lower and upper control limits represent process variation.

Control charts have two general uses in an improvement project. The most common application is as a tool to monitor process stability and control. A less common, although some might argue more powerful, use of control charts is as an analysis tool.

Types of Control Charts:

• Control Charts for Variables
• Control Charts for Attributes

MBA Question:

1) You are Head of a manufacturing unit and want to implement control chart in one of your injection molding shop. Hourly random 500 units are inspected for defects and the average defectives are 3.5%.
a. What type of control chart will you use and what will be its control limits?
b. What are the other types of control chart used in monitoring a process?

2) You are attempting to develop a quality monitoring system using control chart for some parts produced from a large batch production. These parts are either good or defective. During each delivery a random 500 units is sampled and the number of defective units in the sample is noted. The average defect rate is 3.5%.

a. State with reasons the type of control chart you would use (5 Marks)
b. Develop the upper and lower control limits (z=3)

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