Field note · June 30, 2024
Before you clip the outliers
A max 4.1× the 99th percentile, and the three different things that could be.
A tail worth looking at rather than removing. first_response_min on support-tickets tops out at 4,444 against a median of 83.
The max is 4.1× the 99th percentile and 17.9 standard deviations above the mean. The median is 83, so the top of this column lives in a different regime from the middle of it.
select *
from support_tickets
order by first_response_min desc
limit 20;Run that before deciding anything. There are three things an extreme value can be, and they need opposite treatment: a data error, a real but rare event, or a different population that got mixed into the same table. Only the first is safe to drop, and you cannot tell them apart from the summary statistics — you have to look at the rows.
A ratio this large usually means the third case. Something in this column is not the same kind of thing as the rest, and the honest fix is a filter with a stated reason rather than a percentile clip.
Clipping at the 99th percentile is a decision to discard your most interesting rows. Sometimes right. Never automatic, and never silent — if a number in a report was computed on clipped data, the report has to say so.