Accuracy vs. Precision
It is important to understand that measurements (observations) may be precise but not accurate if they are closely grouped together around a value that is different from the expectation ("true" value) by a certain amount. Also, observations may be accurate but not precise if they are well distributed about the expected value but are significantly dispersed from one another. Finally, observations can be both precise and accurate if they are closely grouped around the expected value (the distribution mean).