Outlier: what it means
A value that is unusually far from the rest of the data. It may be a genuine extreme or an error, and it can distort averages and trends.
An outlier is a value that sits far away from most of the other values in a dataset. It may be real, such as one very large company or one exceptional month, or it may be a recording error.
Why it matters when checking a claim
- Averages. A single outlier can move the mean substantially while leaving the median almost unchanged.
- Start and end points. Comparing with an unusual year, such as 2020, can make a change look much larger or smaller than the longer trend. See base effect.
- Local figures. In small areas, one event (such as a single large incident) can produce a rate that looks extreme.
- Producers check for them. The ONS uses outlier detection when compiling statistics such as consumer prices, and bulletins often explain when one-off events have affected a figure.
Also searched as: outliers, outlier meaning, extreme value, anomaly in data
Questions that use this term
Related terms
- Mean: The total of all values divided by how many there are. Often what people mean by 'average', it can be pulled up or down by a few extreme values.
- Median: The middle value when all values are ordered from lowest to highest. Half are above and half below, so a few very large values cannot pull it up.
- Base effect: When a rate of change is pushed up or down by what happened in the comparison period, rather than by what is happening now.
- Rolling average: An average recalculated each period over a fixed window, such as the latest three months or twelve months, to smooth out short-term ups and downs.
More statistics, surveys and quality terms: Accredited official statistics · Official statistics in development · Pre-release access · Survey sample · Response rate · Administrative data · Provisional figures · Revisions · Confidence interval · Statistical significance · Headline figure · Sample size.