There are two key factors in any measuring device:
1. Accuracy - How close is the device's measuremet to a known standard? If you put a 10 gram weight on a scale, what measurement does it give?
It is worth noting that accuracy can be adjusted on many devices.
2. Consistency/Variance. Now here we have the biggie. If you measure the same standard each time, how consistent is the measuement and to what degree does the measurement vary?
Any scale is a good scale if they possess a reasonable degree of accuracy, but they absolutely have to be consistent and have minimal variance. Otherwise, what you're getting isn't a measurement, it's an average.
Grouse
