Chapter 10: Quality Control
Quality Control: is an activity that evaluates quality characteristics relative to a standard, and takes corrective action
when they do not meet standards.
Inspection: appraisal of goods or services against standards.
How Much/How Often: low cost high volume items require little inspection, so automated inspection is used. When
there is high human involvement necessitate more inspection.
Where/When: either in the beginning (do not pay for goods with low quality standards, and in processing low quality
material), at the end of a process (it is cheaper to repair on the field than after the customer buys it), before a costly
operation, before and irreversible operation, before a covering operation.
Centralized vs. On-site: specialized labs are worth the time and cost, so companies rely on self inspections onsite.
Statistical Process Control SPC: Statistical evaluation of the output of a process during production
Quality control steps:
1. Define the quality characteristics to monitor.
2. Measure the characteristics
3. Compare to a standard and evaluate
4. Take corrective action if necessary.
Variations and Control
a. Random variation: Natural variations in the output of process, created by countless minor factors
b. Assignable variation: A variation whose source can be identified
Control Chart: A time ordered plot of sample statistics, used to distinguish between random and non random variability
Control Limits: The dividing lines between random and nonrandom deviations from the mean of the sampling
Type I error: concluding that a process has changed when it has not
Type II error: concluding a process is in control when it is actually not
Designing a control chart:
1. Determine a sample size
2. Obtain 20 to 25 samples
3. Establish preliminary control limits and graph them
4. Plot the sample statistic values on a control chart, and note whether
any points fall outside control limits (CL) 5. If no points outside CL assume there is no assignable cause. If points are outside CL investigate and correct
6. Operators use control chart by recording the value of sample statistic which is periodically tUCL X z
p-Chart - Control chart for attributes, used to monitor the proportion of defectives in a process X z