Statistical Process Control(ppt 40頁)（英文版）

Statistical Process Control內容提要：
Product Specification and Process Variation
Product specification
desired range of product attribute
part of product design
length, weight, thickness, color, ...
nominal specification
upper and lower specification limits
Process variability
inherent variation in processes
limits what can actually be achieved
defines and limits process capability
Process may not be capable of meeting specification!
Process Control Charts
Establish capability of process under normal conditions
Use normal process as benchmark to statistically identify abnormal process behavior
Correct process when signs of abnormal performance first begin to appear
Control the process rather than inspect the product!
Process Control Charts Error Trade-offs
Setting control limits too tight (e.g., ? ± 2??) means that normal variation will often be mistaken as an out-of-control condition (Type I error).
Setting control limits too loose (e.g., ? ± 4??) means that an out-of-control condition will be mistaken as normal variation (Type II error).
Using control limits works well to balance Type I and Type II errors in many circumstances.
3? is not sacred -- use judgement.
Process Control Charts When to Take Action
A single point goes beyond control limits (above or below)
Two consecutive points are near the same limit (above or below)
A run of 5 points above or below the process mean
Five or more points trending toward either limit
A sharp change in level
Other erratic behavior.
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