**1. n. [Formation Evaluation]**

Result of the evaluation aimed at characterizing the range within which the true value of a quantity is estimated to lie, generally with a given likelihood. (ISO) The uncertainty is the amount of possible inaccuracy. It is often a statistical estimate of this range, such as the half-width of a gaussian distribution. The quantity may have been measured or derived from an equation.

**2. n. [Reservoir Characterization]**

The degree to which a dataset may be in error or stray from predicted values. Sometimes quantified in terms of variance or standard deviation, uncertainty exists in data because of a variety of problems, such as poor calibration or contamination or damage to rocks prior to measurement. Uncertainty is the cause of many problems, which occasionally can be overcome by normalizing the data.