First Edition Dated September, 2000 30
Section 4.2.6 When similar key characteristics from different
products are combined on the same control charts (a part or
product family or process output control approach), the
characteristics shall have similar variability and shall be
traceable back to the specific part or product.
When first developing key characteristics, AQS Control Plans and statistical
control charts, a producer often places emphasis strictly on individual part key
characteristics. This emphasis, if continued, can lead to maintaining a large
collection of AQS Control Plans and control charts. Since it is the process
that makes the product “good” or “bad”, limited benefit will be gained from
monitoring and controlling only the product instead of the process. Often
similar key characteristics can be combined on a single AQS Control Plan
and control chart by identifying part groupings or by analyzing the same
feature being produced by the same process on different parts. See D1-9000-
1 for details on how to use process control to reduce the number of charts to
be monitored, while retaining the integrity and diagnostic ability of the
statistical process control charts that are maintained.
The purpose of a control chart is to monitor a single process. If the process
being monitored is actually more than one process, valuable information
regarding the processes will be lost. Control limits will not be calculated
correctly and incorrect decisions will be made. For this reason, a statistical
