outlier
n. an extreme observation or measurement, that is, a score that significantly differs from all others obtained. For instance, assume a researcher administered an intelligence test to a group of people. If most individuals obtained scores near the average IQ of 100 yet one person had an IQ of 150, the latter score would be an outlier. Outliers can have a high degree of influence on summary statistics (e.g., the mean and standard deviation can be pulled severely toward outliers) and on estimates of parameter values, and they may distort research findings if they are the result of error.