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Editorial Team
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Asked: 2026-05-15T00:57:14+00:00

say I have a postgresql table with the following values: id | value ———-

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say I have a postgresql table with the following values:

id | value
----------
1  | 4
2  | 8
3  | 100
4  | 5
5  | 7

If I use postgresql to calculate the average, it gives me an average of 24.8 because the high value of 100 has great impact on the calculation. While in fact I would like to find an average somewhere around 6 and eliminate the extreme(s).

I am looking for a way to eliminate extremes and want to do this “statistically correct”. The extreme’s cannot be fixed. I cannot say; If a value is over X, it has to be eliminated.

I have been bending my head on the postgresql aggregate functions but cannot put my finger on what is right for me to use. Any suggestions?

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  1. Editorial Team

    I cannot say; If a value is over X, it has to be eliminated.

    Well, you could use having and a subselect to eliminate outliers, something like:

    HAVING value < (
 SELECT 2 * avg(value)
 FROM   mytable
 GROUP BY ...
)

    (Or, for that matter, use a more complex version to eliminate anything above 2 or 3 standard deviations if you want something that will be better at eliminating only outliers.)

    The other option is to look at generating a median value, which is a fairly statistically sound way of accounting for outliers; happily there are three reasonable examples of just that: one from the Postgresql Wiki, one built as an Oracle compatability layer, and another from the PostgreSQL Journal. Note the caveats around how precisely/accurately they implement medians.

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