# How many standard deviations to determine outliers

#### mathprof

##### New Member
Are outliers more than two or three standard deviations from the mean?

#### Junes

##### Member
There's no formal definition, it's subjective. But 3 sd seems to be a common cutoff.

#### noetsi

##### Fortran must die
There is no agreed on point of what is an outliers. 3 standard deviations is probably the most common one.

#### hlsmith

##### Less is more. Stay pure. Stay poor.
Yes, I agree there is not a standard definition and I will point out that just based on data distributions some values will be say two or three SDs out. I may hold off saying 3 sds is fairly common, since I only know my own field and because I think fields vary in defining what may be considered extreme - say they may want a 1/million or billion percent.

#### Karabiner

##### TS Contributor
In practice, outliers are defined are those values in a dataset whose removal can
turn an undesired test result ("not significant") into a desired one ("significant").

SCNR

#### hlsmith

##### Less is more. Stay pure. Stay poor.
Agreed, if you are just looking for a value that is say the 99 percentile, well - you are going to find one. On the other hand, if you are looking for a value with a different data generating process or that is erroneous - these are different.

@Karabiner I was kind of thinking of the opposite scenario. Where faulty or different DGP derived observation, removed then makes two groups more comparable or not a different. But as eluded to, there are many definitions and rationales for sleuthing them out.