QUALITY CONTROL AND SPC. CHAPTER 16. DAVID A. COLLIER AND JAMES R. EVANS. 16-1 Describe quality control system and key issues in manufacturing and service. 16-2 Explain types of variation and the role of statistical process control (SPC).
DAVID A. COLLIER AND JAMES R. EVANS
16-1 Describe quality control system and key issues in manufacturing and service.
16-2 Explain types of variation and the role of statistical process control (SPC).
16-3 Describe how to construct and interpret simple control charts for both continuous and discrete data.
16-4 Describe practical issues in implementing SPC.
16-5 Explain process capability and calculate process capability indexes.
arriotthas become infamous for its obsessively detailed standard operating procedures (SOPs), which result in hotels that travelers either love for their consistent good quality or hate for their bland uniformity. “This is a company that has more controls, more systems, and more procedural manuals than anyone—except the government,” says one industry veteran. “And they actually comply with them.” Housekeepers work with a 114-point checklist. One SOP: Server knocks three times. After knocking, the associate should immediately identify themselves in a clear voice, saying, “Room Service!” The guest’s name is never mentioned outside the door. Although people love to make fun of such procedures, they are a serious part of Marriott’s business, and SOPs are designed to protect the brand. Recently, Marriott has removed some of the rigid guidelines for owners of hotels it manages, empowering them to make some of their own decisions on details.
What do you think?
What opportunities for improved quality control or use of SOPs can you think of at your college or university (e.g., bookstore, cafeteria)?
The task of quality control is to ensure that a good or service conforms to specifications and meets customer requirements by monitoring and measuring processes and making any necessary adjustments to maintain a specified level of performance.
If a defect or service error is identified and corrected in the design stage, it might cost $1 to fix. If it is first detected during the production process, it might cost $10 to fix. However, if the defect is not discovered until it reaches the customer, it might cost $100 to correct.
Quality at the sourcemeansthe people responsible for the work control the quality of their processes by identifying and correcting any defects or errors when they first are recognized or occur.
Two basic mistakes when attempting to control a process:
Adjusting a process that is already in control.
Failing to correct a process that is out of control.
Goodman Tire periodically tests its tires for tread wear under simulated road conditions. The company collects twenty samples, each containing three radial tires from different shifts over several days of operations. The overall mean is computed as 31.88, and the average range is 10.8.
Excel Template x-bar and R-Charts
for Goodman Tire Solved Problem
Control limits are:
R-Chart for Goodman Tire Example
Rules for identifying a shift in the process:
8 points in a row above or below the center line.
10 of 11 consecutive points above or below the center line.
12 of 14 consecutive points above or below the center line.
2 of 3 consecutive points in the outer one-third region between the center line and one of the control limits.
4 of 5 consecutive points in the outer two-thirds region between the center line and one of the control limits.
Exhibit Extra Illustration of Some Rules for Identifying Out-of-Control Conditions
Exhibit 16.2 Excel p-Chart Template for Solved Problem
UCLc= c + 3
LCLc= c - 3
Exhibit 16.3 Excel c-Chart Template for Solved Problem
c-Chart for Machine Failures
SPC is a useful methodology for processes that operate at a low sigma level (less than or equal to 3-sigma).
For processes with a high sigma level (greater than 3-sigma), few defects will be discovered even with large sample sizes.
More advanced tools are necessary.
Exhibit 16.4 Process Capability versus Design Specifications
One hundred and twenty measurements of the dimension of a manufactured part for an automobile were taken from a controlled process. The average dimension was 10.7171 and the standard deviation was 0.0868. The upper specification limit is 10.9 and the lower specification limits is 10.5. What are the process capability indexes?
Exhibit 16.5 Portion of Excel Process Capability Template
Exhibit 16.6 Histogram Displayed in the Excel Process Capability Template
Construct and interpret a control chart for the packing slip data.
Conduct a Pareto analysis of the errors.
What information might a separate chart for each error category provide? Conduct a more thorough analysis for each error category, and draw conclusions and suggest recommendations for improving the process.