Imagine this example. I've a field with following values:
10000 13455 90 12347 90 1111 235679
How can I identify that 90 are a outlier? Using average or standard deviation?
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We can plot an outlier box plot with fences to graphically display our data. Then we can define quantities for identifying extreme values in the tails of the distribution. For instance, if
Q1 is the first quartile,
Q3 is the third quartile and the interquartile range is
IQ = Q3-Q1 then
lower inner fence: Q1 - 1.5*IQ upper inner fence: Q3 + 1.5*IQ lower outer fence: Q1 - 3*IQ upper outer fence: Q3 + 3*IQ
Finally, we can apply an outlier detection criteria. A point beyond an inner fence on either side is considered a mild outlier. A point beyond an outer fence is considered an extreme outlier.
See example of an outlier box plot.