Ch. 6 Flashcards
What is Statistical quality control?
- SQC refers to statistical techniques and tools used for sampling, monitoring, and testing the quality of goods and services.
- SQC is divided in three broad categories: (1) descriptive statistics, (2) acceptance sampling (after), and (3) statistical process control (during) (SPC).
- Why should this be important for business graduates? Examples: medical services, food industry, retail industry, safety culture, remember the costs of quality (class 4 = prevention and appraisal).
- Quality is a competitive priority (class 2)
- Quality is integrated in the conception, design, manufacturing and delivery processes (class 3)
Define variability
Variability refers to the causes of variation in a process
*A parallel definition is that quality improvement is the elimination of waste. This is useful in service or transactional businesses.
What are the categorization in variation?
- Common or random (unavoidable)
2. Assignable or systemic (can be identified & eliminated), employee training, machine repair
What are descriptive statistics?
They are used to describe quality characteristics to a product or service.
What is the statistical process control?
Involves inspecting a random sample of output from a process and deciding whether the process is producing products with characteristics that fall within preset specifications.
What is a control chart?
A time ordered plot of sample statistics (e.g. means) obtained from an ongoing process.
What are control charts for variables & for attributes?
Variables: are used to monitor characteristics that can be measured: length, weight, diameter, time, temperature of oven.
Attributes: are used to monitor characteristics that have discrete values and can be counted: % defective, # of flaws in a shirt, rotten apple or not - yes/no
What is the mean (x-bar) chart?
They are used to monitor changes in the mean of a process (central tendency)
What is the range (R) chart?
whereas x bar charts measure a shift in the central tendency of the process, range charts monitor the dispersion or the variability of the process.
ex: volume dispersion
What 2 charts are used together?
Mean x-bar and range R charts
- X-bar would detect inaccurate average temperature.
- R-chart will detect changes in temperature.
- Use R-chart first. If off limits, it means process variation is out of control. Next investigate cause.
- No need to interpret x-bar chart if R-chart out of control.
- If R-chart is in control, then interpret x-bar chart.
- If off limits, then the process average is out of control.
What is a P-Chart & C-Chart?
P-chart: use for quality characteristics that are discrete (attributes) and involve yes/no or good/bad decisions (proportion)
- Number of leaking caulking tubes in a box of 48
- Number of broken eggs in a carton of 18
- Defective or not defective
- Good or bad
- Broken or unbroken
- Black or white Hershey’s kiss chocolates in a bag
C-Chart: use for discrete defects when there can be more than one defect per unit (number)
- Number of flaws or stains in a carpet sample cut from a production run.
- Number of complaints per customer at a hotel.
**Negative number for LCL round to 0
What is the process capability?
It is the ability of the production process to meet or exceed preset specifications.
What are the 3 possible ranges for CP?
CP = 1 process variability just meets specifications
CP<1 process is not capable of producing within specifications
CP>1 process exceeds minimal specifications
Six-sigma companies want to achieve a CP of how much?
2.0
What is the six sigma quality?
The term Six Sigma indicates a level of quality in which the number of defects is no larger than 3.4 parts per million.
Introduced by Motorola
Uses a 6 level of std. deviation
Benchmark for several industries
