ISO 9000 Certification & ISO/TS 16949

Control limits are adjusted too often. If the process is under statistical control and is stable, there is little need to adjust the limits. Limits should only be adjusted if: a) process improvement can be verified, or b) the process inputs have changed, resulting in a change in process capability (as measured by CPk). Also, be cautious of computer programs that automatically adjust control limits based on some pre-defined protocol.

Control limits are incorrectly set. Common errors are: a) forcing limits to be symmetrical about the product nominal value (or some other, arbitrary target value); b) failure to use process data to establish the limits; c) arbitrarily setting the limits to some predetermined values, such as 50% of the product tolerance. (The formulas for establishing control limits do not make use of product specifications!)

A range chart is not used. Some people have the misconception that all that is needed to control a process is to keep the process average centered. This misses the entire issue of piece-to-piece variation.

Points plotted beyond a control limit are ignored. The usual excuses include: a) "I checked the next few parts and they were okay." b) "It was only one point! I never get excited over only one point!" Obviously, the whole concept of assignable cause has been lost in these instances. (A quick method to evaluate the significance of these beyond-limit points is to perform a short CPk analysis. If the CPk value has not changed significantly, the point was a true, statistical "freak". If, however, the CPk value has significantly changed, the point was not a freak and, in fact, the process has changed.)

Very little attention to out-of-control conditions that occur within the control limits. There is common misconception that the only time a process is out-of-control is when a point plots beyond one of the limits. In reality, of several common out-of-control conditions, only one of them is a point beyond a limit. Missed are the "within-limits" out-of-control patterns, such as unexpected trends, unexpected cycles, shifts, stratification, increased variation and few points near the center line.

Charts get completed, filed, but never or only rarely analyzed. This gets back to the issue of management understanding and responsibility. If SPC charts are only used to keep records or to satisfy outside auditors, there is a tremendous loss of valuable data and knowledge that can and should be used as part of the quality improvement process.

Charts are maintained by someone other than the process operator. While there are specific, rare cases where this is the best (and perhaps only) way to implement SPC, such as where laboratory analysis is required, this practice misses the whole point of giving the process operator knowledge, authority, responsibility and ownership of the process. As long as someone else is checking the product characteristic or process parameter, it becomes that person's responsibility to control the process.

SPC training given without regard to the timing of implementation. This is a sure way to waste your training dollars: train in large groups; implement whenever it is convenient, even if it is weeks or months later. By then, most, if not all, of the new-found knowledge will have been forgotten.

Applying SPC to the process, but measuring the wrong characteristic or parameter or measuring the right characteristic or parameter but at the wrong place. This practice sends the absolutely wrong message to the workers. It states loud and clear that management does not understand SPC nor the process. Having the operator measure something over which he or she has absolutely no control is senseless and downright frustrating.

Mixing more than one process on one chart. The most common situation occurs on machines with multiple stations, such as a multiple-head drill press. Even though this may appear to be one machine, each spindle must be thought of as a separate process. The same issue applies to multiple cavity molds.

Control limits are never reviewed and adjusted. This is almost as bad as adjusting the limits too often. The process will change. These changes must be monitored and their effects evaluated. Monitoring should be performed by someone very knowledgeable in SPC. Based on the analysis, charts may be added or deleted; sample frequency may be adjusted. CAUTION: All apparent changes to the process should be validated before any changes are made to the control charting process, limits, etc.

Using SPC charts without control limits. This is little more than record keeping. It is okay if, in fact, you are in the early phases of an SPC application and you are just gathering data. However, if management believes that this is an adequate and acceptable way to use "statistics", they have missed the point completely.

The process is set up well off target. This is a certain way to assure that the process will "run out-of -control". It also demonstrates a lack of discipline and understanding of what the limitations of an SPC chart are and what is required to properly use and interpret a control chart.

Limits are based upon product specification. This is a very common error, generally made with good intentions. The idea is to force the target to be the nominal specification value and set the limits at, say, 50% of the product tolerance. Of course, this misses the whole point that SPC is based on controlling the capability of the actual process, not wishful thinking.

Corrective action not noted & recorded. This is a fatal flaw. All that knowledge of what was done to correct a problem is lost. No one but the person involved in the corrective action benefits. It is permissible to "code" typical causes and corrective actions, provided some sort of analysis (pareto?) is periodically performed to determine if there are any recurring problems.

Download PDF Version: Linking ISO 9001:2000 Requirements to Product/Service Quality

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