Control Charts in Manufacturing Process

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Questions and Answers

What is the primary function of a control chart?

To determine process variability

To select the process and set tolerance limits

To judge whether statistical control has been attained or not (correct)

To evaluate product quality level

Which of the following is NOT a purpose of a control chart?

To reduce scrap or percentage rejection

To improve product design (correct)

To select the process and set tolerance limits

To detect unusual variations in a process

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

To indicate the mean of the statistic (correct)

To indicate the standard error

To indicate the upper control limit

To indicate the lower control limit

What is the difference between control charts for variables and attributes?

Variables are continuous while attributes are discrete Signup and view all the answers

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

To provide an early warning of potential problems Signup and view all the answers

What is the purpose of annotation with events of interest in a control chart?

To provide information about the process Signup and view all the answers

What is the main difference between variable control charts and attribute control charts?

Variable control charts involve the measurement of job dimensions, while attribute charts only differentiate between defective and non-defective items. Signup and view all the answers

What is the advantage of attribute control charts?

They allow for quick summaries of product quality and are easily understood by managers. Signup and view all the answers

What is the benefit of using control charts for attributes over variables?

They are less time-consuming 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 based on sampling. Signup and view all the answers

What is the ultimate goal of using control charts in a production process?

All of the above Signup and view all the answers

What type of chart is used to analyze the fraction of defectives?

P-chart Signup and view all the answers

What is the term used to describe variable control charts according to Montgomery (1985)?

Leading indicators of trouble Signup and view all the answers

At which stages is inspection carried out for acceptance?

At various stages, including inspection of incoming materials, process inspection, final inspection, and inspection of the finished product by the purchaser. Signup and view all the answers

What is the term used to describe the use of sampling inspection to decide whether or not to accept a lot?

Acceptance sampling Signup and view all the answers

Why are attribute control charts used in certain cases?

Because measurement of quality characteristics is either not possible or it is very complicated and costly to do so. Signup and view all the answers

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

To prevent unnecessary frequent adjustments to the process Signup and view all the answers

What is the characteristic of a control chart that allows it to detect variations in the processing?

It is based on statistical sampling theory Signup and view all the answers

What should be done when a process is currently under control?

Use data from the process to predict future performance Signup and view all the answers

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

Degraded process performance Signup and view all the answers

What is the purpose of analysis of a control chart when the process is not in control?

To determine the sources of variation and eliminate them Signup and view all the answers

What is a characteristic of a control chart that distinguishes it from a run chart?

It allows significant change to be differentiated from natural variability Signup and view all the answers

What should be done when some products show variation outside the specified tolerance limits?

Take a review of the situation and take corrective action Signup and view all the answers

What is the benefit of using control charts in a manufacturing process?

It enables correct decisions regarding control of the process Signup and view all the answers

What is the primary 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 reason why Acceptance Sampling is widely used in practice?

It is less expensive than 100 percent inspection Signup and view all the answers

What is the effect of inspection fatigue involved in 100 percent inspection?

It removes 82 to 95% of defective material Signup and view all the answers

What is the purpose of performing inspection at various stages?

To monitor the quality of goods Signup and view all the answers

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

To show the probability of accepting 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 producer's risk in Acceptance Sampling?

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

What is the purpose of a sampling plan?

To specify the parameters of the sampling process and the acceptance/rejection criteria Signup and view all the answers

What is the primary difference between single sampling and double sampling plans?

Double sampling involves two stages of inspection, whereas single sampling involves only one Signup and view all the answers

What is the main advantage of using sequential sampling plans?

It allows for more accurate decision-making Signup and view all the answers

What is the primary disadvantage of using sequential sampling plans?

They involve higher administrative costs Signup and view all the answers

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

A second sample is drawn Signup and view all the answers

What is the minimum number of samples required in a sequential sampling plan?

3 Signup and view all the answers

What is the purpose of the acceptance number (C1) in a double sampling plan?

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

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

A fifth sample is drawn Signup and view all the answers

What is the primary characteristic of a sequential sampling plan?

It allows for additional samples to be drawn until a decision can be made 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.

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Description

This quiz covers the basics of control charts, a statistical tool used to monitor and control manufacturing processes. Learn how control charts help prevent unnecessary adjustments and detect variations in processing.