In statistical process control, understanding the source of variation in a process is critical for determining whether the process is stable and predictable. In statistical process control, random variationsare also known as assignable variationsshould be larger than the allowance for variation provided by design or customer specifications for a process to be capablelead to occasional false conclusions that processes are out of controlare variations that can be traced to a specific causeare variations that are investigated and removed before estimating process capability to ensure the process is stable