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.