dep_delay_min: mean 10, median 5

A number worth keeping: on flight-delays, dep_delay_min has a mean of 10 and a median of 5.

The mean is 10. The median is 5. That is a gap of 98.7%, and it is not noise — the p90 is 28 and the max is 478, so the top of the distribution is dragging the average somewhere a minority of rows actually live.

select
  avg(dep_delay_min)                                                     as mean,
  percentile_cont(0.5) within group (order by dep_delay_min)             as median,
  percentile_cont(0.9) within group (order by dep_delay_min)             as p90,
  max(dep_delay_min)                                                     as max
from flight_delays;

The practical consequence is that "average dep delay min" answers a question nobody asked. If someone wants to know what a typical row looks like, the median answers it. If someone is forecasting a total, the mean is the right tool and the skew does not matter, because the mean times the count is the total.

So the rule is not "never use the mean". It is: name which question you are answering, then pick the statistic that answers it. A report that shows 10 with no median next to it has quietly decided you meant the first question.

There is a longer version of this in Describing a column without lying, and the pattern is the short one.