When measuring a defined length with a ruler, there is a source of uncertainty and the measurement may need estimation or rounding between two points. When doing this estimation, it is possible to over estimate and under estimate the measured value, meaning there is a possibility for random error. Also, the ruler itself may be too short or too long causing a systematic error. For example, the illustration to the right shows a pencil whose length lies between 25cm and 26cm. With an intermediate mark, the ruler shows in greater detail that the pencil length lies somewhere between 25.5cm and 26cm. Therefore, one may reasonably approximate that the length of the pencil is 25.7cm. The presence of a systematic error, however, would likely be more subtle than a random error because the environment may affect the ruler in a difficult to notice way or the ruler itself may have slightly inaccurate markers.

Precision vs. Accuracy

Precision is often referred to as reproducibility or repeatability. For example, consider the precision with which the golf balls are shot in the figures below. A set of shots that are only precise would mean you are able to cluster your shots near each other on the green but you are not reaching your goal, which would be to get the golf balls into the hole. This concept is illustrated in the left picture of the two figures below. Accuracy, on the other hand, is how close a value is to the true or accepted value. The picture to the right demonstrates