What is the best practice when you have zero defects?
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.