Miranda, Consultant, Netherlands, Member
I would say that you could check the data using a histogram
. If the RC mentioned does not appear regularly / more than once in a larger period of time, it would most likely be a special cause variation.
Next to that I would recommend to always ask the employees who participate in the process
: if they have been working within that process for quite some time, they will be able to judge and advise if it's common cause or special cause variation.
To statistically see the difference, you can use a control chart (minitab)
Outliers (above or under the ucl/lcl) need to be investigated and prevent it from happening again.
Note: process should be stable and predictable in order to recognise valid outliers.
Hope this helps, good luck!