It is not simply a cumulative count of all kids ever served. We don't believe in telling donors how many kids we served once upon a time - we'd like everyone to know how many kids are actively drinking safe water.

It goes up and down. If one of our systems fails, the number of children served is reduced by that number. When the system comes back online, the number goes up again. We don't count the kids unless they are drinking clean water.

It counts kids only - even though we know that adults benefit from our installations, too. Teachers, medical staff, and janitors are all examples of the adults who benefit from our work. We do track those numbers internally, so we can have an accurate gauge of use. But we publicly display only numbers that count the children who benefit.

It subtracts when systems are de-commissioned. Why? Because they aren't needed anymore. A good example of this is when we provided water purification systems for earthquake relief in China. When refugees were finally resettled, there was no need for our systems in their camps. We stopped counting them (which is actually a good story, not a bad one).