The calculations for capability analysis are based on the following assumptions:

1. The data is normally distributed . In other words, the shape shown by the histogram follows the "normal" bell curve .
   
   
2. The system being studied is stable and no assignable causes for variation are present. A control chart of the system has been made to determine stability before a capability analysis is done.
   
   
3. The mean of the system being studied falls between the upper and lower specification limits defined for the process.

If these assumptions are not met, the results of a capability analysis will be misleading.

See also:
>> Can a process produce output within specifications?
>> Capability vs control
>> Non-normal data capability analysis
>> What is capability analysis and when is it used?
>> What are the capability indices?
>> Learning more about capability