Chapter 9

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## Chapter 9

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**Control Chart Decision Tree**Defectives Defects**What is a p chart?**• An attribute data control chart • Used to chart fraction defective or proportion nonconforming data • The subgroup size can be fixed or it can vary**How a p chart is used**• Used to establish performance control of any process: • Pass / fail situations • Percent defective per sample • Good for any technical or administrative process**11 step procedure for control charts**• Select a process measurement • Stabilize process and decrease obvious variability • Check the gages (10:1, GRR) • Make a sample plan • Setup the charts and process log • Setup the histogram • Take the samples and chart the points • Calculate the control limits and analyze for control • Calculate the capability and analyze for capability • Monitor the process • Continuous Improvement**p chart control limits**• Chart the points • Sets scales for control chart • Calculate each subgroup’s proportion nonconforming • Plot the proportion nonconforming on the chart • Calculate control limits and analyze for control • Plot your control limits**p chart 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 < USL or LCL > LSL then Cpk>1**Advantages of a p chart**• Data readily available • Helps prioritize problem areas • Easy to create with fixed and variable subgroup sizes**Disadvantages of a p chart**• 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**p Chart Example**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?