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10.stastistical process control attributes (10.3 preparing and …
10.stastistical process control attributes
10.1 type of data
variable
(thing we measure)
lenght
weight
time
blood pressure
attribute
(thing we counted)
defective
part/product inspected not conforming or meeting specification (can have > 1 defects)
unit as a whole , not acceptable or does not meet performance requiment
monitored using p n np chart
defect
items that contribute to non conformity of a quality characteristics
irregullarity or problem with larger unit , countable
mornitored using c and u
10.2 understand the control chart for attribute data
variable sample size
defect : U- CHART
samples are taken and the total number of nonconformance in the sample is determined
tracks the average number of nonconformance per unit
defective :P-CHART
a single chart that tracks the percentage of nonconforming item in each sample
sample sizes are large, usually 100 pieces or more
constant sample size
defect: C-CHART
graphs the total number of nonconformance found in each piece or unit that is inspected
defective : NP- CHART
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
10.3 preparing and interprating the control chart
Construction of p Chart (variable subgroup size)
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
Construction of c chart
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
Construction of u chart (count of defects per unit)
• For unit > 1
• For varying sample size
• Also if subgroup size constant
Construction of p Chart (constant subgroup size)
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
10.4 selecting the right control chart