The control charts in SOM are classified as either variable or attribute charts. Variables are measurements on a continuous scale such as inches or pounds. What types of variables can be monitored with the variables control charts? Anything that can be measured and expressed in numbers, such as temperature, dimension, hardness number, tensile strength, weight, viscosity, etc. Variables are monitored in the MR-CHART worksheet for the mean and range of samples and in the I-CHART for individual observations. Attributes are discrete data such as the number of items in the sample that are defective or the number of defects in one unit of product. The P-CHART and CU-CHART models are available for attributes data.
Operations Manager
I-CHART Overview
The I-CHART is another variables control chart used to monitor individual observations (samples of one each) rather than larger samples. The I-CHART is most used in cases where a considerable period of time elapses between opportunities to collect quality observations. For example, there only be one quality observation per shift in operations with long lead times. Although the I-CHART is usually identified with manufacturing applications, it is widely used in casinos to monitor card, dice, and roulette games. The individual observations entered in the chart are typically the net cash profits recorded when a dealer settles his or her account with the house at the end of a work period.
P-CHART is the most versatile and popular control chart. To use P-CHART, quality inspectors classify sample items into two groups: good or bad. This can mean defective or non-defective, conforming or non-conforming to specifications, acceptable or unacceptable, or other definitions in which there are only two categories of results.
Brookshire Cookware Corporation, a producer of pots and pans located in Brookshire , Texas , uses P-CHART worksheets to monitor the quality of component parts received from its Mexican suppliers. At the end of each business day, Brookshire faxes a copy of each P-CHART to its suppliers. This feedback helps maintain a good working relationship with suppliers and alerts them to potential problems.
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