An outlier is a question first

A value far from the rest of a collection can be an error, an unusual but real event, or a clue that several different groups have been mixed. Its appearance alone does not settle the explanation.

Inspect the original record and the method that produced it. Check units, entry rules, and surrounding observations. Avoid removing a value solely because the chart becomes tidier without it.

If a value is excluded for a justified reason, record the rule and consider how the result changes with it included. An unusual observation becomes useful when the investigation is transparent enough for someone else to understand the decision.