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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  1. Select quality characteristic
  1. Determine subgroup size and method size subgroup depends on p
  1. Collect the data
  1. Calculate the trial limits
  1. Establish revised limits
  1. Achieve the objective

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  1. Select quality characteristic
  1. Determine subgroup size and method size subgroup depends on p
  1. Collect the data
  1. Determine trial limits
  1. Establish revised central line and control limits

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  1. Select the quality characteristics
  1. Determine the subgroup size and method : For c chart n = 1
  1. Collect data, calculate trial limits
  1. Draw the centerline and control limits on the chart
  1. Interpret the chart
  1. Revising the c chart

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• For unit > 1

• For varying sample size

• Also if subgroup size constant