Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
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?