The usefulness of measurement is enhanced by knowledge of its level of certainty. Multiple measurements of the same property are like multiple shots at the same target. The pattern of the shots tells you something about the measurement and its ability to describe the 'true' value of the property being sought. The patterns above depict possible outcomes of different experiments to measure the same propert. Expt IV is of course the best, because it give very reproducible results (precise) and also results that are very close to the true value or bullseye (accurate). Experiment III is precise but not accurate. It exhibits systematic error, which is insidiously difficult to estimate at times.