Often manufacturing operations use a single part at the beginning of the day to verify that the process is targeted. If the part measured is off target, the process is then adjusted. Later, in some cases another part is measured and again the process may be adjusted. Dr. Deming referred to this type of measurement and decision-making as tampering.(Funnel Experiment)
Consider a situation where the weight of a precious metal coating on a part is being controlled to a target of 5.00 grams. Suppose that the results from the scale used to determine the weight vary +-0.20 grams but this is not known since the measurement system analysis was never done. The operating instructions require the operator to verify the weight at setup and every hour based on one sample. If the results are beyond the interval 4.90 to 5.10 grams then the operator is to setup the process again.
At setup, suppose the process is operating at 4.95 grams but due to measurement error the operator observes 4.85 grams. According to instructions the operator attempts to adjust the process up by .15 grams. Now the process is running at 5.10 grams for a target. When the operator checks the setup this time, 5.08 grams is observed so the process is allowed to run. Over-adjustment of the process has added variation and will continue to do so.
This is one example of the funnel experiment that Dr. Deming used to describe the effects of tampering. The measurement error just compounds the problem.
Four rules of the funnel experiment are:
Rule 1: Make no adjustment or take no action unless the process is unstable.
Rule 2: Adjust the process in an equal amount and in an opposite direction from where the process was last measured to be.
Rule 3: Reset the process to the target. Then adjust the process in an equal amount and in an opposite direction from the target.
Rule 4: Adjust the process to the point of the last measurement.
The setup instruction for the precious metal process is an example of Rule 3. Rules 2, 3 and 4 add progressively more variation. Rule 1 is the best choice to produce minimum variation.
Other examples of the funnel experiment are:
Recalibration of gages based on arbitrary limits – i.e., limits not reflecting the measurement system’s variability. (Rule 3)
(Re)mastering the process control measurement system after an arbitrary number of uses without any indication or history of a change (special cause). (Rule 3)
Auto compensation adjusts the process based on the last part produced. (Rule 2)
On the job training (OJT) where worker A trains worker B who later trains worker C… without standard training material. Similar to the “post office” game. (Rule 4)
Parts are measured, found to be off target, but when plotted on a control chart the process is shown to be stable – therefore, no action is taken. (Rule 1)
source : Analysis of measurement systems
