If, instead of measuring the amount of gas in the tank, the gas gauge showed the octane rating of the gasoline, it would not help you decide when to refill the tank. Likewise, a report card that measured the number of pencils used by the student would be a poor indicator of academic performance. Understandable An indicator must be understandable. You need to know what it is telling you. There are many different types of gas gauges.
Some gauges have a lever that moves between 'full' and 'empty' marks.
Other gauges use lights to achieve the same effect. Some gauges show the number of gallons of gasoline left in the tank. Although different, each gauge is understandable to the driver. Similarly, with the report card, different schools have different ways of reporting academic progress. A through F. Other schools use numbers from 100 to 0.