The measurement of a parameter have been assigned to fall within a certain range, say 40 to 80. We call this the control range. If a measurement is made where any value falls outside this control range then it might be an indication that there is something wrong with either the measurement or the device making the measurement.

To ensure that the device is operating under peak conditions we specify to the user of the device that we expect the value of the measurement to fall within "80% of the control range". If it doesn't then check the device for problems.

As far as I'm concerned this implies that the value must fall within a range of 44 to 76.

Is it necessary for me to specify to the user that the measurement should fall within "the central 80% of the control range"?

Why would someone think that I may really want a different range that still encompasses 80% of the above control range, like 42 to 74? I would think that the portion of the control range that I 'm inquiring about is implied.