Field note · July 9, 2024
Before you clip the outliers
A max 1.6× the 99th percentile, and the three different things that could be.
A tail worth looking at rather than removing. wind_mw on grid-energy-load tops out at 4,200 against a median of 589.
The max is 1.6× the 99th percentile and 6.7 standard deviations above the mean. The median is 589, so the top of this column lives in a different regime from the middle of it.
select *
from grid_energy_load
order by wind_mw 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 modest is consistent with an ordinary heavy tail — nothing anomalous, just a distribution that is not symmetric.
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.