Defects Defectives

 An attribute data control chart  Used to chart fraction defective or proportion nonconforming data  The subgroup size can be fixed or it can vary

 Used to establish performance control of any process: ◦ Pass / fail situations ◦ Percent defective per sample ◦ Good for any technical or administrative process

Select a process measurement 2. Stabilize process and decrease obvious variability 3. Check the gages (10:1, GRR) 4. Make a sample plan 5. Setup the charts and process log 6. Setup the histogram 7. Take the samples and chart the points 8. Calculate the control limits and analyze for control 9. Calculate the capability and analyze for capability 10. Monitor the process 11. Continuous Improvement

Chart the points ◦ Sets scales for control chart ◦ Calculate each subgroup’s proportion nonconforming ◦ Plot the proportion nonconforming on the chart 8. Calculate control limits and analyze for control ◦ Plot your control limits

Calculate the capability and analyze for capability ◦ Capability is based on average defective ◦ Is UCL or LCL within your goal value?  If UCL > USL or LCL < LSL then Cpk<1  If UCL LSL then Cpk>1

 Data readily available  Helps prioritize problem areas  Easy to create with fixed and variable subgroup sizes

 Counts failures - not prevention  Cannot isolate causes of process problems  Variable control limits may be confusing on variable subgroup sizes  Limits hard to construct on variable subgroup sizes

10 This data represents the number of errors found in purchase orders over a 30 week period. 1. Complete a p chart. 2. What can you tell from the data? 3. Complete a np chart. 4. What can you tell from the data? 5. How are these charts different?

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