Tenths instead of hundredths
Deciles do for data what percentiles do, at a coarser grain: nine cut points, D1 to D9, that split a sorted dataset into ten groups of equal size. D1 is the 10th percentile, D5 is the median, D9 the 90th percentile. The coarse grain is a feature. Income and wealth statistics are reported in deciles because the stories live in the tenths: what the bottom decile earns against the top decile is a one-line summary of a distribution's shape that no mean can give, and it stays stable where individual percentiles jump around.
Twenty monthly savings amounts, sorted from 41 to 95. The cut points come out at D1 = 57.7, D2 = 62.4, D3 = 67.4, D4 = 71.2, the median D5 = 74.5, rising to D9 = 89.3. Read as a sentence: the least-saving tenth of months ended below 57.7, half the months ended below 74.5, and the top tenth saved more than 89.3. The calculator above also locates any single value: 85 has percentile rank 77.5 in this data (15 values below it, one equal), which places it in decile 8.
Deciles, quintiles, quartiles: one family
All of these are quantiles at different resolutions: quartiles cut at quarters, quintiles at fifths, deciles at tenths, percentiles at hundredths. The computation never changes: sort, find the position with the percentile method, interpolate. Choosing the grain is a communication decision: the more granular, the more the cut points wobble in small samples. With 20 values, deciles put just two observations in each group, which is why published decile tables come from large surveys.
Saying "the fifth decile group" when you mean D5 the cut point. Deciles name cuts; the groups between them are tenths. Precision here prevents the classic off-by-one band error in reports.
Frequently asked questions
What is the 5th decile?
The median: the cut point with 50% of the data below it. D5, the 50th percentile and the second quartile are the same number.
How many groups do deciles create?
Ten groups (tenths) from nine cut points. Each group holds about 10% of the data.
Deciles or percentiles: which should I report?
Deciles for summaries aimed at non-specialists and grouped comparisons; percentiles when a precise position matters, as in test scoring.