MGMT 330 Ch 17 Quality Control and SPC

Quality Control

Ensures that a good or service conforms to specifications

Any control system has three components:

A performance standard or goal.

A means of measuring actual performance.

Comparison of actual performance with the standard to form the basis for corrective action.

Quality at the Source

means the 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.

Quality at the Source

means the 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.

Supplier Certification and Management

If incoming materials are of poor quality, then the final manufactured good will certainly be no better. Suppliers should be expected to provide documentation and statistical evidence that they are meeting required specifications.

In-Process Control

ensure that defective outputs do not leave the process and, more important, to prevent them in the first place.

Finished-Goods Controlled

focused on verifying that the product meets customer requirements. For many consumer products, this consists of functional testing.

In the context of the net promoter score (NPS), which of the following is a difference between promoters and detractors?

Unlike promoters, detractors defect at higher rates

Variation occurs for many reasons, such as

inconsistencies in material inputs, changes in environmental conditions, machine maintenance cycles, customer participation and self-service, tool wear, and human fatigue.

Help prevent sources of errors and mistakes by:

− Using the poka-yoke approach

− Hiring and training service providers in service management skills

− Measuring customer satisfaction

− Using the net promoter score (NPS) metric that gives a difference in the percentage of promoters and detractors

Common Cause Variation

the result of complex interactions of variations in materials, tools, machines, information, workers, and the environment.

Special (or assignable) Cause Variation

arises from external sources that are not inherent in the process, appear sporadically, and disrupt the random pattern of common causes.

In control

when no special causes affect the output of a process.

Out of control

when special causes are present.

Statistical Process Control (SPC)

a methodology for monitoring the quality of manufacturing and service-delivery processes to help identify and eliminate unwanted causes of variation.

Control Chart

a run chart to which two horizontal lines, called control limits are added: the upper control limit (UCL) and lower control limit (LCL).

Overadjusting a process that is in control will:

increase the variation in the output

Construction and Use of Control Charts

Preparation, Data Collection, Determination of Trial Control Limits, Analysis and Interpretation and Use as a Problem-Solving Tool.

Preperation

− Choose the metric to be monitored and controlled

− Determine the sample size and frequency of sampling

− Set up the control char

Data Collection

− Record the data

− Calculate relevant statistics

− Plot the statistics on the chart

Determination of Trial Control Limits

− Draw the center line (process average) on the chart

− Compute the upper and lower control limits

Analysis and Interpretation

− Investigate the chart for lack of control

− Eliminate out-of-control points

− Recompute control limits if necessary

Problem-Solving Tools

− Continue data collection and plotting

− Identify out-of-control situations, and take corrective action

Continuous Metric

one that is calculated from data that are measured as the degree of conformance to a specification on some continuous scale of measurement.

Discrete Metric

one that is calculated from data that are counted.

x chart

a control chart that monitor the centering of the process.

R chart

a control chart to monitor the variation in the process.

A process is in control when the control chart has the following characteristics:

No points are outside control limits.

The number of points above and below the center line is about the same.

The points seem to fall randomly above and below the center line.

Most points, but not all, are near the center line, and only a few are close to the control limits.

When a process is out of control, we typically see some unusual characteristics. The most common are:

A point outside the control limits.

A shift in the average value.

A gradual trend.

A cyclical pattern.

Some typical rules that are used to identify a shift include:

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, and

4 of 5 consecutive points in the outer two-thirds region between the center line and one of the control limits.

p chart

a control chart that monitors the proportion of nonconforming items

c chart

monitors the total number of nonconformances per unit when the size of the sampling unit or number of opportunities for errors is constant.

In determining whether a process is in statistical control, the _____ is always analyzed first.

R chart

A television manufacturer wants to monitor the proportion of defective television sets that do not meet quality standards from a sample of 100 television sets. In the context of quality control charts, the manufacturer is most likely to use a(n) _____.

p chart

In the context of control charts, the _____ is used to control the total number of nonconformances per unit when the size of the sampling unit or number of opportunities for errors is constant.

c chart

Designing control charts involves two key issues:

Sample size and sample frequency

SPC is a useful methodology for processes that operate at

three sigma level or less.

In the context of designing control charts, which of the following is true of small sample sizes?

They are desirable to keep the cost associated with sampling low.

Process Capability

refers to the natural variation in a process that results from common causes.

Process Capability Study

a carefully planned study designed to yield specific information about the performance of a process under specified operating conditions.

Typical questions that are asked in a process capability study are:

Where is the process centered?

How much variability exists in the process?

Is the performance relative to specifications acceptable?

What proportion of output will be expected to meet specifications?

Process Capability Index

the relationship between the natural variation and specifications is often quantified by a measure.

Process Capability Index formula (C_p )

Upper Specification Limit - Lower Specification Limit / 6 * standard dev