Measurement, precision and accuracy

Precision and accuracy answer two different questions, and physics keeps them strictly separate even though everyday speech doesn't. Precision asks: if I measure the same thing again, will I get the same number? Accuracy asks: is that number actually close to the true value?

A bathroom scale that reads 3 kg high every single time is highly precise — every measurement agrees with the last — but not accurate, since every one of them is wrong by the same amount. A dartboard analogy makes it vivid: darts clustered tightly together but far from the bullseye are precise but not accurate; darts scattered randomly but centred on the bullseye on average are accurate but not precise.

The two failures need different fixes. Poor precision (random scatter) is improved by repeating the measurement and averaging. Poor accuracy from a consistent offset — a systematic error — isn't fixed by repetition at all; it needs the source of the offset to be found and corrected, such as re-zeroing that bathroom scale.