Estimating Uncertainties on a single measurement

Depends on instrument used and how this instrument is used.

• Precision of a graduated instrument is at least 1/2 of the smallest division, but
• calibration of the zero position,
• parallax, and
• instruments drift or fluctuations all contribute to increase the final uncertainty.

Common sense must always be used in order to evaluate uncertainties. This process will become easier as you build experience, that's why it is important to do it systematically, for every measurement you make.