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the quantitative aspects of quality management
statistical quality control (SQC)
variation that is caused by factors that can be identified and managed
assignable variation
variation that is inherent in the process itself
common variation
What variation can be controlled
assignable variation
What variation is also called random variation
common variation
the center points of a set of numbers (average)
mean (x-bar)
a measure of how much individual observations deviate from the mean (spread)
standard deviation (sigma)
the maximum acceptable value for a characteristic
upper specification
the minimum acceptable value for a characteristic
lower specification
the ability of a process to consistently produce a good/deliver a service with a low probability of generating a defect
process capability
range of variation that is considered acceptable by the designer or customer
specification limits
range of variation that a process is able to maintain with a high degree of certainty
process capability
ratio of the range of values allowed divided by the range of values produced; shows how well the parts being produced fit into the range specified by the design specifications
process capability index (Cpk)
Cpk larger than one means:
the process is capable
When the two numbers of the Cpk are not close, it means:
the mean has shifted
If both Cpk are under one it means:
the distribution is TOO wide
testing a sample of output to determine if the process is producing items within a preselected range
statistical process control
quality characteristics that are classified as either conforming or not conforming
attributes
characteristics that are measured using an actual value
variable
what is the average of p hat?
p bar
p bar=
Total # defective units from all samples / (# samples x sample size)
UCL =
p bar + zSp
LCL=
p bar - zSp
How do you create a p chart? (5 steps)
Calc sample proportions for each sample (p)
Calc avg of sample proportions (p bar)
Calc the stand dev of sample proportion (Sp)
Calculate the control limits (UCL and LCL)
Plot the individual sample proportions, avg of proportions, and control limits
p charts are used when:
an item or service is either food or bad (yes no decision)
c charts are used when:
an item or service may have multiple defects
c bar =
avg number of defects per unit
x bar charts show
aim target value
r bar charts show
range
for x bar and r charts, size of sample should be around:
5 units
for x bar and r charts, number of samples should be around:
25 samples
for x bar and r charts, frequency of samples should be around:
trade off between cost of sampling and benefit of adjusting system
for x bar and r charts, control limits should be around:
z=3 (unless told otherwise)
performed on goods that already exist to determine what percent of the products conform to specifications; determines quality level and ensure quality is within predetermined level; executed through sampling plan
acceptance sampling
What are the three results possible from acceptance sampling?
Accept
Reject
Retest
What are the disadvantages of acceptance sampling?
Risk of accepting bad lots and rejecting good lots
Added planning and documentation
Sample provides less info than 100 percent inspection
What are the advantages of acceptance sampling?
Economy
Less handling damage
Fewer inspectors
Upgrading of the inspection job
Applicability to destructive testing
Entire lot rejection (motivation for improvement)
maximum acceptable percentage of defectives defined by producer
acceptable quality level (AQL)
percent of defectives that defines consumers rejection point
lot tolerance percent defective (LTPD)
the probabiility of rejecting a good lot
producer risk (alpha)
The probability of accepting a bad lot
consumer risk (beta)
the AQL in Six sigma states that ___ defective parts out of every 1,000,000 parts might slip through the system
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