In statistics, an outlier is an observation that is numerically distant from the rest of the data. Grubbs defined an outlier as:

An outlying observation, or outlier, is one that appears to deviate markedly from other members of the sample in which it occurs.

Outliers can occur by chance in any distribution, but they are often indicative either of measurement error or that the population has a heavy-tailed distribution. In the former case one wishes to discard them or use statistics that are robust to outliers, while in the latter case they indicate that the distribution has high kurtosis and that one should be very cautious in using tools or intuitions that assume a normal distribution. A frequent cause of outliers is a mixture of two distributions, which may be two distinct sub-populations, or may indicate 'correct trial' versus 'measurement error'; this is modeled by a mixture model.

In most larger samplings of data, some data points will be further away from the sample mean than what is deemed reasonable. This can be due to incidental systematic error or flaws in the theory that generated an assumed family of probability distributions, or it may be that some observations are far from the center of the data. Outlier points can therefore indicate faulty data, erroneous procedures, or areas where a certain theory might not be valid. However, in large samples, a small number of outliers is to be expected (and not due to any anomalous condition).

Malcolm T. Gladwell, CM (born September 3, 1963) is a Canadian journalist, bestselling author, and speaker. He is currently based in New York City and has been a staff writer for The New Yorker since 1996. He has written four books, The Tipping Point: How Little Things Make a Big Difference (2000), Blink: The Power of Thinking Without Thinking (2005), Outliers: The Story of Success (2008), and What the Dog Saw: And Other Adventures (2009), a collection of his journalism. All four books were New York Times Bestsellers.

Gladwell's books and articles often deal with the unexpected implications of research in the social sciences and make frequent and extended use of academic work, particularly in the areas of sociology, psychology, and social psychology. Gladwell was appointed to the Order of Canada on June 30, 2011.

Gladwell was born in Fareham, Hampshire, England to Joyce, Jamaican-born psychotherapist, and Graham Gladwell, a British mathematics professor. Gladwell has said that his mother is his role model as a writer. When he was six his family moved to Elmira, Ontario, Canada.