Specification Limits are targets based on the customer requirements or expectations. The Voice of the Customer is the customer’s specification limits. They are different than control limits, which are calculated according to the control chart you are using. Specification limits are the targets set for the process/product by customer or market performance or internal target. In short it is the intended result on the metric that is measured. Control limits on the other hand are the indicators of the variation in the performance of the process. It is the actual values that the process is operating on. It is the real time value.
Suppose an athlete wants to participate in 200 m race in Olympics. He approaches a coach and asks him to guide him for the same. The Coach, will show him the recent best performance records and tell that, “In order to win the game, you need to run at a target speed of 20- 25 seconds”. Because, 20s-25s is the speed of the winning participants observed from the previous games. Now, our athlete starts his practice and his performance is tracked. His performance ranges from 22s – 30s (3 Sigma deviations from the average performance). Now the target is 20-25 seconds whereas the actual performance is 18s to 26s.