What is the best practice when you have zero defects?

Matt SMatt S PQ Systems Employee

I recently had a conversation with a customer who was collecting defect data each shift and charting it as a percent. The “problem” was that for this chart, she had ten consecutive days with zero defects. The thirty samples (10 days x 3 shifts/day), shows a flat line and no control limits. Since there is no variability in the data, the standard deviation is 0, therefore control limits can’t be calculated. She further explained that they are “required” to chart this data. What would you recommend she do in this case?

I can think of a few options such as:

1.       Ignore the ‘requirement’ to chart this since charting this does not add value.

2.       Change the time period how defects are counted. For example instead of each shift, count them each day.

3.       Show the control chart with no control limits and all of the data at 0.

4.       Do something else.

 Please share your suggestions.


Comments

  • since you are required to chart this data. I would do number 3- Show the control chart with no control limits and all of the data at 0. Problems may creep in if you aren't checking this data.
     
  • You may have "zero defects" but you still have variation in the process and in the component and/or subcomponents. What is the risk of not continuing to reduce variation?
  • Matt SMatt S PQ Systems Employee
    Tim said:
    You may have "zero defects" but you still have variation in the process and in the component and/or subcomponents. What is the risk of not continuing to reduce variation?
    I agree, that variation should be reduced until the point at which it does not make economical sense. In her case, she was counting the number of defective items per shift and her operational definition concluded that there was nothing defective. She was not measuring, for example, cylindricity, but rather the part "fit."
  • Without knowing what the process is and how it's measured I can only guess.
    1) Does the measurement system have the precision and accuracy at least 4x greater than the tolerance limits? If the variation is less than the accuracy of the measurement instrument it's possible to have the same reading every time. (Example tolerance/range of process is ± .0002" and a caliper accurate to ± .001" is used).
    2) Are the values recorded in the full decimal place? If the variation is low, say ± .0004" and the spreadsheet is set up for 2 decimal places it will always be the same value.
    3) Did they perform a GR&R and is the NDC value greater than 5?
    4) Have people performing the input been trained on what's expected?
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