Outlier Handling Decision Guide
Decide to keep, fix, or remove extreme values.
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Help me decide what to do with outliers. Variable and context: [VARIABLE_AND_CONTEXT]. Summary and the extreme values: [SUMMARY_AND_EXTREMES]. What the analysis will be used for: [USE_CASE]. Use this decision tree and show the path: 1. Is the value impossible (negative age, a date in the future)? If yes, treat it as an error: correct from the source, set to missing, or exclude, and record it. 2. Is it possible but suspicious (a unit error such as grams versus kilograms, a typo with an extra zero)? Look for a pattern and test the hypothesis. 3. Is it real and rare (a large customer, a record sale)? Keep it, and consider robust methods (median, trimmed mean, winsorizing, log transform) or reporting results with and without it. 4. Does it drive the conclusion? Run a sensitivity analysis. Provide the detection code (IQR rule: below Q1 - 1.5 x IQR or above Q3 + 1.5 x IQR; z-scores; domain limits) with a warning that these rules flag values, not errors. Output a table with Value | Flag reason | Classification | Decision | Justification, and a sentence for the methods section describing what was done. Never remove a point only because it spoils the result.
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Data: delivery times in minutes; most 20-60, a handful over 600. Questions: Are they errors (typed 6000 instead of 60)? Rare but real events (storms)? Different process (custom orders)? Decision table Cause | Action | Document Data entry error | Correct or set to missing | Rule and count Real extreme | Keep; use robust summaries (median, trimmed mean) | Note Different population | Analyse separately | Reason Never delete values just because they hurt your result. Sensitivity: report results with and without the 7 extreme rows. Plot: boxplot plus a histogram on log scale.
Illustrative only, made-up data. Your results will differ and must be reviewed.