Outlier Calculator

Compute the interquartile range IQR = Q3 − Q1, then the fences: lower = Q1 − 1.5×IQR and upper = Q3 + 1.5×IQR. Any value below the lower fence or above the upper fence is an outlier.

It is John Tukey’s convention from exploratory data analysis, wide enough that normal data rarely gets flagged (about 0.7% would be, for a normal distribution) but tight enough to catch genuinely unusual values. Some fields use 3×IQR for “extreme” outliers.

Not automatically. First check whether the value is a data-entry error, a measurement problem, or a real observation. Real outliers often carry the most interesting information, report them, and if you exclude them, say so.

Yes, values with |z| > 2 or 3 are often flagged. But z-scores use the mean and standard deviation, which outliers themselves distort. The IQR rule resists that, which is why box plots use it.