The exclusive method: the data is split at the median, and if n is odd, the median itself is excluded from both halves before finding Q1 and Q3 as the medians of the lower and upper halves. This matches the method taught in most GCSE and A-Level statistics courses — other valid methods can give slightly different Q1/Q3 for the same data, which is expected, not an error.
What is the 1.5×IQR rule?+−
The standard convention for flagging potential outliers: any value below Q1 − 1.5×IQR or above Q3 + 1.5×IQR is flagged. This calculator applies it automatically and lists any flagged values.
Why does the box plot use the five-number summary instead of the mean and standard deviation?+−
The five-number summary (min, Q1, median, Q3, max) is resistant to outliers, since quartiles only depend on position in the sorted data, not the magnitude of extreme values. This makes the box plot a more robust visual summary than one built from the mean and SD when outliers are present.
Is a flagged outlier definitely a data error?+−
No — a flagged value needs investigation, not automatic removal. Some outliers are genuine data-entry errors; others are real, informative extreme values that belong in the analysis. The 1.5×IQR rule identifies candidates for a closer look, not a verdict.