Rules for interpreting control charts

T

Thoops76

#1
Hi All,
I have some questions with regards to SPC. Any help that you guys on the cove can give would be very much appreciated.

1. What are the rules for interpreting control charts? Specifically attribute control charts. Do the rules differ for attribute / variable charts, or do the same rules apply to both?

2. Does anyone have an example of a P chart. Including the steps to take when working on the raw data to completing the graph.

3. Finally, what are the pro's and con's of using attribute charts when compared to using variable charts.

Thanks in advance.

Regards... Tony
 
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Marc

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#2
I'm not sure what you mean by 'rules' for interpreting charts. You might want to take a look at Statsoft Textbook as a starter.

p chart - Existing p Chart discussion threads.

You might want to look at SPC p Chart form in .xls Excel and p Charts on Excel.

A p-chart is an attributes control chart used with data collected in subgroups of varying sizes. Because the subgroup size can vary, it shows a proportion on nonconforming items rather than the actual count. P-charts show how the process changes over time. The process attribute (or characteristic) is always described in a yes/no, pass/fail, go/no go form. For example, use a p-chart to plot the proportion of incomplete insurance claim forms received weekly. The subgroup would vary, depending on the total number of claims each week. P-charts are used to determine if the process is stable and predictable, as well as to monitor the effects of process improvement theories.

p-chart.gif


Advantages of attribute control charts: Attribute control charts have the advantage of allowing for quick summaries of various aspects of the quality of a product, that is, the engineer may simply classify products as acceptable or unacceptable, based on various quality criteria. Thus, attribute charts sometimes bypass the need for expensive, precise devices and time-consuming measurement procedures. Also, this type of chart tends to be more easily understood by managers unfamiliar with quality control procedures; therefore, it may provide more persuasive (to management) evidence of quality problems.

Advantages of variable control charts: Variable control charts are more sensitive than attribute control charts (see Montgomery, 1985, p. 203). Therefore, variable control charts may alert us to quality problems before any actual "unacceptables" (as detected by the attribute chart) will occur. Montgomery (1985) calls the variable control charts leading indicators of trouble that will sound an alarm before the number of rejects (scrap) increases in the production process.
 

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