Learning Objectives
LO 13s-1: Describe the types of process variations and the measurements required for quality control purposes.
Natural variations are uncontrollable process variations. Assignable variations are process variations that can be traced to a specific “fixable” cause. Variable data is continuous process data such as weight and time. Attribute data is process data that indicate the presence of some attribute such as color, satisfaction, etc.
LO 13s-2: Construct the process control charts for variable data.
See figure 13S.2, x chart, page 419, and figure 13S.3, R chart, page 420. There are two types of control charts for measuring and plotting variable data: the x chart and the R chart. The x chart is used to track the central tendency; the R chart is used to track sample ranges. For both charts, 25–30 samples of sizes 5–10 must be collected. For each sample, the mean and range are calculated. Next, the mean of all samples and the mean of sample ranges are calculated. These measures are used as the center line of their applicable chart. The means are also used to calculate the upper control limits and the lower control limits for each chart.
LO 13s-3: Construct the process control chart for attribute data.
See figure 13S.4, P chart, page 422. P charts are the control charts for attribute data. P charts monitor the percent defective in each sample. The center line of a P chart is the mean percent of all samples collected. Upper and lower control limits are calculated.
LO 13s-4: Calculate the process capability index and show it is used.
The process capability index, Cpk, shows whether the process mean has shifted away from the design target and is off-center. Calculating the index shows how well a process is performing. If the Cpk is greater than 1.0, the process is deemed capable of meeting the design specifications.
LO 13s-5: Explain how acceptance sampling is used.
Acceptance sampling is used to determine the quality of an entire shipment of goods. When shipments of goods are received from suppliers or before goods are shipped to customers, samples are taken from the shipment and compared to a quality acceptance standard. The quality of the sample is then assumed to represent the quality of the entire shipment. The concern is that the smaller the sample size, the greater the risk of incorrectly judging a shipment’s quality.
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