Outlier Detection Methods in Excel
Spot unusual values and decide what to do.
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Help me find outliers in a column of numbers. Description of the data: [DATA_DESCRIPTION]. What an unusual value might mean in this context: [CONTEXT]. Show two methods with formulas: the interquartile range rule and the z-score, and explain when each fits. Provide steps to flag outliers with conditional formatting and to list them in a separate table. Describe how to investigate a flagged value (check entry errors, units, duplicates, genuine extremes) and a decision log with action taken. Warn against deleting values just because they are inconvenient, and suggest reporting results with and without outliers when they matter. Use an invented example of twenty values with two flagged. Add a section on documenting the decision for each outlier, with the original value kept in a separate column, so that the analysis can be audited. Add a short example showing how the two methods can disagree on a value and what that tells me about the distribution of the data.
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Invented values: 12, 14, 15, 15, 16, 17, 18, 19, 21, 55. Q1 = 15, Q3 = 18.75, IQR = 3.75. Fences: about 9.4 and 24.4. The value 55 is outside. Z-score: mean 20.2, standard deviation about 12.5; z for 55 about 2.8. Decision log: 55 checked against source; entry error (should be 15.5). Corrected; original kept in a column. Note: methods can disagree on borderline values. Reporting: present the summary with and without the corrected value, and note that the correction was an entry error, not a removal of an inconvenient result.
Illustrative only, made-up data. Your results will differ and must be reviewed.