Capability Index Essay

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The capability index shows how well the parts being produced fit into the range specified by the design specification limits. If the specification limits are larger than the three sigma allowed in the process, then the mean of the process can allowed to drift off-center before readjustment, and a high percentage of good parts will still be produced. The capability index is the position of the mean and tails of the process relative to design specification. The more off-center, the greater the chance to produce defective parts. Because the process mean can shift in either direction, the direction of the shift and its distance from the design specification set the limit on the process capability. The direction of shift is toward the smaller number.

Measurement by attributes means taking samples and using a single decision—whether the item is good or bad. P charts are used for process control with attribute measurements. Simple statistics are used to create a p chart with an upper process control limit and a lower process limit. The process is assumed to be working correctly when the samples, which are taken periodically during the day, continue to stay between the control limits. The size of the sample must be large enough to allow counting the attribute. X and R charts are widely used in statistical process control. In attribute sampling, we determine whether something is good or bad, fits or doesn’t fit. X and R charts are used for process control with variable measurements. In variables sampling, we measure the actual weight, volume, number of inches, or other variable measurements, and we develop control charts to determine the acceptability or rejection of the process based on those measurements. There are four main issues to address in creating a control chart: the size of the samples, number of samples, frequency of samples, and control limits. For industrial applications in
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