Control Charts in Quality Control

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary purpose of a control chart?

To eliminate all sources of variation in the process

To predict the future performance of the process without analysis

To make frequent adjustments to the manufacturing process

To detect variations in the processing and warn if there is any departure from the specified tolerance limits (correct)

What is the basis of control charts?

Process control parameters

Randomized experiments

Quality control principles

Statistical sampling theory (correct)

What action should be taken when a process is in control?

Stop the process immediately

Analyze the chart for further variations

Leave the process alone (correct)

Adjust the process control parameters

What is the result of adjusting process control parameters for a process that is in control?

Degraded process performance Signup and view all the answers

What is the purpose of analyzing a control chart?

To determine the sources of variation in the process Signup and view all the answers

What is a control chart?

A specific kind of run chart Signup and view all the answers

What happens when a review of the situation is taken in control charts?

Corrective steps are taken immediately Signup and view all the answers

What is the objective of an objective and disciplined approach to control charts?

To enable correct decisions regarding control of the process Signup and view all the answers

What do variable control charts involve?

Measuring the job dimensions and accepting or rejecting items based on tolerance limits Signup and view all the answers

What is the purpose of acceptance sampling?

To decide whether to accept or reject a production lot of material Signup and view all the answers

What kind of charts are more sensitive than attribute control charts?

Variable control charts Signup and view all the answers

What is the advantage of attribute control charts?

It allows for quick summaries of various aspects of the quality of a product Signup and view all the answers

When is attribute control charts the only way to control quality?

When measurement of quality characteristics is not possible or is very complicated and costly Signup and view all the answers

What kind of charts are used for process control?

(X-Bar) and R charts Signup and view all the answers

What is inspection for acceptance?

The use of sampling inspection to decide whether or not to accept the lot Signup and view all the answers

What is the purpose of inspection in acceptance sampling?

To decide whether to accept or reject the lot Signup and view all the answers

What is the purpose of Acceptance Sampling?

To make a decision as to whether to accept or to reject lots based on inspection of sample(s) Signup and view all the answers

What is the main advantage of Acceptance Sampling over 100% inspection?

It is less expensive Signup and view all the answers

What is the Operating Characteristic Curve (OC Curve) used for?

To show the probability that you will accept lots with various levels of quality Signup and view all the answers

What is the Acceptance Quality Level (AQL)?

The maximum % defective that can be considered satisfactory as a process average for sampling inspection Signup and view all the answers

What is the Lot Tolerance Percent Defective (LTPD)?

The level of quality routinely rejected by the sampling plan Signup and view all the answers

What is the Producer's risk in Acceptance Sampling?

The risk of rejecting a good lot Signup and view all the answers

What is the Consumer's risk in Acceptance Sampling?

The risk of accepting a defective lot Signup and view all the answers

What is a sampling plan?

A plan for sampling inspection of goods Signup and view all the answers

What is a control chart primarily used for?

To judge whether statistical control has been attained or not Signup and view all the answers

What is the basis of a single sampling plan?

Drawing a sample from a bigger lot Signup and view all the answers

What is the purpose of a center line in a control chart?

To indicate the mean of the statistic Signup and view all the answers

What happens if the number of defectives in the first sample is between C1 and C2 in a double sampling plan?

A second sample is drawn Signup and view all the answers

What are the two types of characteristics that can be measured by control charts?

Variables and attributes Signup and view all the answers

What is a characteristic of a control chart for variables?

It can be measured and has a continuum of values Signup and view all the answers

What is a characteristic of a sequential sampling plan?

Three or more samples of stated size are permitted Signup and view all the answers

What happens if the lot is neither accepted nor rejected by the end of the fourth sample in a sequential sampling plan?

A fifth sample of n5 is drawn Signup and view all the answers

What is the purpose of upper and lower control limits in a control chart?

To indicate the threshold at which the process output is considered statistically 'unlikely' Signup and view all the answers

What is a disadvantage of a sequential sampling plan?

Higher administrative costs Signup and view all the answers

What is the purpose of a control chart in a manufacturing process?

All of the above Signup and view all the answers

What is a characteristic of a control chart for attributes?

It can be evaluated with a simple yes or no decision Signup and view all the answers

What happens if there are more than r1 defectives in the first sample of a sequential sampling plan?

The lot is rejected Signup and view all the answers

What is the purpose of the second sample in a double sampling plan?

To make a final decision on the lot Signup and view all the answers

What is an optional feature of a control chart?

Upper and lower warning limits Signup and view all the answers

What happens if there are no more than c1 defectives in the first sample of a sequential sampling plan?

The lot is accepted Signup and view all the answers

Study Notes

Control Charts

Control charts are used to determine when to leave a process alone and prevent unnecessary adjustments.
They are graphical representations based on statistical sampling theory.
Control charts detect variations in the processing and warn if there is any departure from specified tolerance limits.
They help in detecting the cause of undesired variations and removing them.
Control charts are used to predict the future performance of a process if it is currently under control.
They can help determine the sources of variation and eliminate them to bring the process back into control.

Purpose and Advantages of Control Charts

Control charts indicate whether a process is in control or out of control.
They determine process variability and detect unusual variations.
They ensure product quality level.
They warn in time, reducing scrap or percentage rejection.
They provide information about the selection of process and setting of tolerance limits.
Control charts build up the reputation of the organization through customer satisfaction.

Components of a Control Chart

Points representing a statistic (e.g., mean, range, proportion) of measurements of a quality characteristic in samples.
The mean of the statistic using all the samples.
A center line drawn at the value of the mean of the statistic.
Upper and lower control limits (natural process limits) indicating the threshold at which the process output is considered statistically 'unlikely'.
Optional features: upper and lower warning limits, division into zones, and annotation with events of interest.

Types of Control Charts

Control charts can be used to measure any characteristic of a product.
Characteristics can be divided into two groups: variables and attributes.
Control charts for variables are used to monitor characteristics that can be measured and have a continuum of values.
Control charts for attributes are used to monitor characteristics that have discrete values and can be counted.

Comparison of Control Charts for Variables and Attributes

Variables charts involve the measurement of job dimensions and an item is accepted or rejected based on tolerance limits.
Attribute charts differentiate between defective and non-defective items without measuring dimensions.
Variables charts are more detailed and contain more information.
Attribute charts require a bigger sample size and are less expensive.
Variables charts are more sensitive and can alert to quality problems before they occur.

Commonly Used Charts

(X-Bar) and R charts for process control.
P chart for analysis of fraction defectives.
C chart for control of number of defects per unit.

Acceptance Sampling

Acceptance sampling is concerned with the decision to accept or reject a mass of manufactured items based on standards of quality.
It uses statistical sampling to determine whether to accept or reject a production lot of material.
Acceptance sampling is usually done as products leave the factory or within the factory.
The decision to accept or reject is made by determining the number of defective items in a sample from the lot.
The lot is accepted if the number of defects falls below the acceptance number or otherwise rejected.

Operating Characteristic Curve

The Operating Characteristic Curve (OC Curve) shows the probability of accepting lots with various levels of quality.
It is the working plan of acceptance sampling.
AQL (Acceptance Quality Level) is the maximum % defective that can be considered satisfactory as a process average for sampling inspection.
RQL (Rejectable Quality Level) is the % defective, also known as the Lot Tolerance Percent Defective (LTPD).

Risks in Acceptance Sampling

Producer's risk: the risk of rejecting a good quality lot due to a bad sample.
Consumer's risk: the risk of accepting a bad quality lot due to a good sample.

Acceptance Sampling Plans

A sampling plan specifies the parameters of the sampling process and the acceptance/rejection criteria.
Variables to be specified include the size of the lot, the size of the sample inspected, the number of defects above which a lot is rejected, and the number of samples that will be taken.
Types of sampling plans include single sampling, double sampling, and sequential sampling.
Single sampling plan: decision is made after drawing a single sample from the lot.
Double sampling plan: a second sample is drawn if the number of defectives in the first sample lies between two acceptance numbers.
Sequential sampling plan: additional samples are drawn until an inference can be made, with a maximum number of samples specified.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz explores the concept of control charts in quality control, including their purpose and advantages. Learn about the device that helps determine process variability and detects unusual variations.