Chapter 10 - Quality Control
In the context of quality control inspection, define the following: - Variables - Attributes
- Variables: characteristics of product/service that can be measured on a continuous scale. - Attributes: characteristics of product/service that can be physically counted.
In the context of quality control, what type of processes are signified by the following acronyms? - IQC - IPQC - OQC
- IQC: Incoming Quality Control (checks quality of inputs for productions/service delivery process) - IPQC: In-Process Quality Control (checks the quality work-in-process during production/service) - OQC: Outgoing Quality Control (checks quality of outputs of production/service delivery process)
In using quality control charts, when is a process considered to be: - In Control - Out of Control
- In Control: when statistic is within control limits, output is acceptable and process is in control. - Out of Control: when statistic is outside control limits, process is out of control and corrective action is needed.
How are the following types of control charts for variables used? - Mean Control Charts - Range Control Charts
- Mean Control Charts: used to monitor and control central tendency of process output. (x bar) - Range Control Charts: used to monitor and control dispersion of resource for process output. (R charts)
Briefly describe the following types of quality control inspection. - On-Site Inspection - Centralized Inspection
- On-Site Inspection: 1) Inspection done as part of production/service delivery process. 2) used when operation requires quick decision or is using/moving to source quality. - Centralized Inspection: 1) inspection done during or after production/service delivery process. 2) used when quality control requires specialized or costly resource such as people or equipment.
From a statistical process control perspective, define the following: - Process Distribution - Sampling Distribution
- Process Distribution: actual distribution of sample statistic values for process output. - Sampling Distribution: theoretical distribution of sample statistic values if variation is random.
In terms of process capability assessment, define the following: - Process Variability/Process Width - Specifications/Specification Width - Process Capability
- Process Variability/Process Width: inherent variability in process output measured in standard deviation. - Specifications/Specification Width: the acceptable range of variation in process output expressed in limits or tolerances. - Process Capability: comparison of inherent variability to an acceptable range of output variation. (process vs specification Width)
What is quality control?
- Process of evaluating output relative to a standard and determining when to take corrective action. - Goal is to achieve/assure quality of conformance.
From a statistical process control perspective, define the following: - Sample Statistic - Sample Size
- Sample Statistic: variable or attribute measured by inspection process and compared to standard. - Sample Size: number of observations per sample collected during inspection process.
What is quality assurance?
process that provides confidence that all quality requirements of enterprise will be fulfilled.
What general approach to quality control is considered the least progressive?
quality control is limited to before and after.
When using control charts, what is the purpose of run tests?
run test used to look for what type variation.
What does the Central Limit Theorem say about sampling distribution?
sample statistic values will become more normally distributed as number of samples increase
What are the key questions in quality control inspection?
1) where to inspect in production/delivery process 2) how much to inspect in terms of outputs 3) whether to inspect on-site or centrally 4) whether to inspect variables or attributes
What is the PDSA Cycle and what does its acronym mean?
Plan-Do-Study-Act Cycle - technique that provides framework for both process improvement and problem solving.
In terms potential errors when using control charts, what is a Type I Error?
chart's control limits lead to belief that process is out of control when it is in control. ( control limits set too close to standard/mean)
Which general approach to quality control is considered the most progressive?
continuous improvement
How is the decision to inspect on-site or centrally generally made?
involves studying tradeoffs between cost of: - process disruption if inspection is done centrally. - inspection resources if inspection done on-site.
What is the most commonly assumed sampling distribution in statistical process control?
normal distribution
In the context of quality control charts, what is sigma?
number of standard deviations from process mean that represents dividing line between random and non-random variation.
In terms of how much and how often to inspect, what is the optimal level of inspection?
optimal level is point at which total cost of inspections and passing defects is at its lowest.
What is DMAIC and what does its acronym mean?
Define, Measure, Analyze, Improve, Control (DMAIC) - technique that provides framework for both process improvement and problem solving associated with six sigma.
How does quality assurance differ from quality control?
- both involve monitoring, measuring and comparing process output to a standard to identify defects. - terms often used interchangeably, but there significant differences between the two. *QA also involves identification and achievement of enterprise's overall quality goals and requirements.
How does Taguchi Cost (Loss) Function differ from common belief regarding unnecessary costs?
- common belief: there is no unnecessary cost if output stays within control limits - to eliminate those costs, Taguchi advocated for improving process capability so that variation from target specification is always zero.
What is Quality Function Deployment (QFD)?
- ensures that all quality requirements of customers are fully understood and met. - integrates voice of the customer into design process using tools like house of quality matrix
How are the following types of control charts for attributes used? - p-Charts - c-Charts
- p-Charts: used to monitor and control proportion of defective items in process output (binomially distributed). - c-Charts: used to monitor and control number of defects per unit of process output. (poisson distribution)
What is process capability assessment?
- process for determining whether production/service delivery process is capable of output within a specific of values. - requires that process is stable which means all variation is output is random variation.
What are the general approaches to quality control?
1) quality control is limited to before and after. - often involves use of acceptance sampling and is least progressive approach to quality control. 2) quality control takes place before, after and during production/service delivery process. - Often involves use of statistical process control. 3) as part of continuous improvement, production/service delivery processes are designed/redesigned to always achieve required quality of output. - the most progressive approach to quality control.
What are the key steps in the quality control?
1) Define what needs to be controlled. 2) Determine how to measure it. 3) Establish standard for it. 4) Create process for comparing results to standard. 5) Determine when process should be found to be out of control due to deviations from standard. 6) Identify how to initiate corrective action when process is determined to be out of control. 7) Determine how long to monitor results of corrective action to ensure that it was effective.
What are some basic approaches to process capability improvement?
1) Simplify 2) Standardize 3) Mistake Proof 4) Upgrade 5) Automate
What are the limitations of process capability measures?
1) if process output is not normally distributed, any measurement of capability will be incorrect. 2) if process is not stable, any measurement of process capability will be meaningless. 3) if process capability ratio used when process mean is not centered, results may be misleading.
What are the potential benefits of process capability improvement?
1) lower quality failure costs. 2) reduced quality control inspection costs. 3) improved productivity and customer satisfaction.
What are the two measurements of process capability and when is each of them used?
1) process capability Ratio (Cp) - used when process mean is centered between upper and lower specifications limits. 2）process capability Index (Cpk) - used when process mean is not centered.
What is quality control inspection?
Appraisal activity that compares the output of production/service delivery process tp standard.
What is the BPI and what does its acronym mean?
Business Process Improvement - a technique for achieving process improvement thru documentation, measurement and analysis. (PI)
In terms of quality control, what is Statistical Process Control (SPC)?
a process that controls quality by monitoring variations from standards using statistical sampling.
In terms potential errors when using control charts, what is a Type II Error?
chart's control limits lead to belief that process is in control when it is out of control. (control limits set too far from standard/mean)
From a statistical process control perspective, what is non-random variation?
variation from standard for which a specific cause can be identified.
From a statistical process control perspective, what is random variation?
variation from standard fro which no specific cause can be identified.