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Operations management chapter 10
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Terms in this set (53)
statistical quality control
uses statistical techniques & sampling to monitor & test the quality of goods & services.
Acceptance sampling
determines to accept or reject a product
Statistical process
control determines if process is operating within acceptable limits.
the best companies quote
The best companies design quality into the process, thereby reducing the need for inspection/tests.
least progressive to most progressive phases of quality control
1 acceptence sampline
2 statistical process control
3 continous improvement and six sigma
what happens when inspection activities increase
if inspection activities increase, inspection costs increase, but the costs of undetected defects decrease. The goal is to minimize the sum of these two costs.
SPC (statistical process control) Steps
1. Take periodic samples from process
2. compare to predetermined limits
3. If outside limits, stop process and take corrective actions
4. if inside limit continue process
assignable variation
A variation whose source can be identified
random variation
Random variation: Natural variations in the output of process, created by countless minor factors
PURPOSE of a control chart
to monitor process output to distinguish between random and assignable variation
control limits are calculated for:
Control limits are calculated for 2 (with 95.5 percent probability for Normal distribution) or 3 (with 99.7 percent probability for Normal distribution) standard deviations of the process mean.
Type 1 error
concluding that a process has changed when it has not
type 2 error
concluding a process is in control when it is actually not
alpha equals
probability of a type 1 error
5 steps to design a control chart
1. determine a sample size
2. obtain 20 to 25 samples
3. establish and graph preliminary control limits
4. plot sample statistic values on control chart
5. are any points outside control limits
-- if yes then investigate and correct
sample mean
used to monitor the mean of a process.
x-bar charts
sample range
used to monitor the process dispersion
R charts
z
Standard Normal deviate (usually z = 3)
grand mean =
X
Average of sample means = grand mean
o
Process standard deviation
0x
Standard deviation of sampling distribution of sample means = o/square root of n
A2
can be obtained from table
_
R
Average of sample ranges
sample range
max. value-min. value in sample
Do this problem
Upper and Lower Control Limits for Sample Range Control Chart Formula
upper and lower control limits for sample mean chart formula
Do this problem
moving range control chart formula
...
moving range (MR)
difference between consecutive observations and used to monitor dispersion or spread when n=1
p chart
control chart used to monitor the proportion of defectives in a process
c chart
control chart used to monitor the number of defects per unit
when to use a p chart
When observations can be placed into two categories.
When observations can be placed into two categories.
when to use a c chart
Use only when number of occurrences per unit of measure can be counted;non-occurrences cannot be counted.
control charts for attributes
design specs
Range of acceptable values established by engineering design or customer requirements
process variability
natural variability in a process
process capability
process variability relative to specification
process capability- if the process is centered and if the process is not formula
process capability ration
specification width/ process width
capability example
capable equals
process output falls within specifications
process capability analysis
process capability example
process capability answer to example
what does a Cpk of .4253 mean
an index that shows how well the units being produced fit within the specification limits--- process considered capable if Cpk is more than or equal to 1
six sigmas want a what for Cpk
2.0 at least
most companies look for a value of Cpk
1.3 or better
design of experiments example
concept check
concept check 2
concept check 3
concept check 4
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