• Errors may be characterized as random, systematic, or blunders – Random error represents the effect of unpredictable variations in the instruments, the environment, and the observing procedures employed – Systematic error represents the effect of consistent inaccuracies in the instruments or in the observing procedures – Blunders or mistakes are typically caused by carelessness and are detected by systematic checking of all work through observational procedures and methodology designed to allow their detection and elimination