Outlier is an observation which seems to deviate markedly from the other members of the sample in which it happens. In the set of systolic blood pressures, {125, 128, 130, 131, 198}, for instance, 198 might be considered an outlier. More formally the term refers to the observation which seems to be inconsistent with the rest of the data, relative to a supposed model. Such extreme observations might be reflecting some abnormality in the measured characteristic of the subject, or they might result from an error in the measurement or recording.