Field note · December 6, 2024

The top of temp_c is 1.4× the 99th percentile

A max 1.4× the 99th percentile, and the three different things that could be.

1 min read ·Statistics ·statistics quality

temp_c on sensor-telemetry: p99 is 49.95 and the max is 71.74.

The max is 1.4× the 99th percentile and 9.3 standard deviations above the mean. The median is 41.28, so the top of this column lives in a different regime from the middle of it.

sql
select *
from sensor_telemetry
order by temp_c 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.