A decile divides a dataset into ten equal parts, each containing 10% of the values when the data are ordered from lowest to highest. They are a type of quantile used to describe the distribution of a dataset, similar to quartiles and percentiles, but split into ten groups instead of four or one hundred.
The nine decile cut points (D₁ through D₉) separate the data into ten segments:
- D₁ is the 10th percentile (10% of values fall below it)
- D₂ is the 20th percentile
- …
- D₉ is the 90th percentile (90% of values fall below it)
Each decile provides a reference point that helps you understand how values in the dataset are spread and where individual data points fall in relation to the rest. Note that the 5th decile (D5) is the median.
Deciles are commonly used in fields like education, finance, and social sciences to rank or categorize individuals or items based on their relative standing.
For example, a standardized test score report includes deciles to show how a student’s score compares to others. If a student’s score falls in the 7th decile, they scored higher than at least 60% of test-takers, but not more than 70%.
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