You run a quartile formula on your data and share the report. A colleague checks the same numbers in R and gets different results. Another uses a TI-84 calculator and gets a third set. All three are correct — they just use different methods.
Excel exposes both major approaches through two functions. QUARTILE.INC uses the inclusive method, matching R, Python, and the legacy QUARTILE function. QUARTILE.EXC uses the exclusive method, matching academic textbooks and TI calculators. Choosing the wrong one doesn’t break your spreadsheet, but it produces numbers that don’t match your team’s expectation.
This guide covers the full syntax, the quart 0–4 support table (including why QUARTILE.EXC returns #NUM! for quart = 0 and 4), and eight practical examples: side-by-side comparison on the same dataset, IQR calculation, outlier detection with the 1.5×IQR fence rule, the five-number summary, salary band classification, box-and-whisker chart preparation, academic grade analysis, and the relationship between QUARTILE and PERCENTILE functions.