10.stastistical process control attributes
10.1 type of data
variable (thing we measure)
attribute (thing we counted)
lenght
defective
weight
time
defect
blood pressure
10.2 understand the control chart for attribute data
part/product inspected not conforming or meeting specification (can have > 1 defects)
10.3 preparing and interprating the control chart
variable sample size
items that contribute to non conformity of a quality characteristics
irregullarity or problem with larger unit , countable
unit as a whole , not acceptable or does not meet performance requiment
monitored using p n np chart
mornitored using c and u
constant sample size
defect: C-CHART
defective : NP- CHART
defect : U- CHART
defective :P-CHART
10.4 selecting the right control chart
- samples are taken and the total number of nonconformance in the sample is determined
- tracks the average number of nonconformance per unit
- a single chart that tracks the percentage of nonconforming item in each sample
- sample sizes are large, usually 100 pieces or more
- graphs the total number of nonconformance found in each piece or unit that is inspected
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- tracks the number of nonconforming items in each sample
- easier to use than the p chart because the percentage of defective items does not have to be calculated, but it has one restrictions
- all the samples must be the same size
Construction of p Chart (variable subgroup size)
Construction of c chart
Construction of u chart (count of defects per unit)
Construction of p Chart (constant subgroup size)
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- Select quality characteristic
- Determine subgroup size and method size subgroup depends on p
- Collect the data
- Calculate the trial limits
- Establish revised limits
- Achieve the objective
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- Select quality characteristic
- Determine subgroup size and method size subgroup depends on p
- Collect the data
- Determine trial limits
- Establish revised central line and control limits
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- Select the quality characteristics
- Determine the subgroup size and method : For c chart n = 1
- Collect data, calculate trial limits
- Draw the centerline and control limits on the chart
- Interpret the chart
- Revising the c chart
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• For unit > 1
• For varying sample size
• Also if subgroup size constant