Bill McNeese
Involved In Discussions
Wondering what others may think. I never have liked the practice of setting a lower control limit to zero for an attribute chart when that lower control limit is negative. I always simply say there is no lower control limit. There is a lower boundary of course - but not a lower control limit.
For example, you are charting occurrences (such as OSHA recordable injuries per month) using a c control chart. The average is 4. That gives a lower control limit of -2. Can't have negative injuries, so there should be on lower control limit - not setting it to 0.
A control limit is "special" in that it marks the realm of common cause of variation. In this example, there is nothing special about 0. You can easily have a month with no injuries and there is nothing "special" about it - although a lot of managers like people were being more careful that month.
So, I prefer to say there is no lower control limit for an attribute chart when it is less than zero. Not sure how the practice of setting it to zero started.
For example, you are charting occurrences (such as OSHA recordable injuries per month) using a c control chart. The average is 4. That gives a lower control limit of -2. Can't have negative injuries, so there should be on lower control limit - not setting it to 0.
A control limit is "special" in that it marks the realm of common cause of variation. In this example, there is nothing special about 0. You can easily have a month with no injuries and there is nothing "special" about it - although a lot of managers like people were being more careful that month.
So, I prefer to say there is no lower control limit for an attribute chart when it is less than zero. Not sure how the practice of setting it to zero started.