A quintile is a 1/5th (20 percent) portion of the whole. In statistics, it’s a population or sample divided into five equal groups, according to values of a particular variable. It can also be used to refer to the cut-of points (quartiles) for these portions.
- 0 – 20% is the first quintile (also called the bottom quintile),
- From 20 to 40% is the second quintile,
- From 40 to 60% is the third quintile,
- From 60 to 80% is the fourth quintile,
- From 80 to 100% is the fifth quintile (also called the top quintile).
Part one: Dividing data into fifths.
The procedure is a little like how you divide data to find a median.
Example problem: Sort the following list of interest incomes into quintiles: 500, 1, 12, 13, 200, 400, 580, 610, 5, 7.
- Place the numbers in order: 2, 4, 7, 12, 13, 250, 350, 550, 580, 610.
- Divide into fifths. You have 10 numbers here, so each fifth will have two items. I’m uses a bar (|) to separate the fifths:
2, 4, | 7, 12, | 13, 250, | 350, 550, | 580, 610.
Part two: Finding contribution percentages for parts of the whole.
Now let’s say you wanted to find out how much the top fifth of incomes contribute to overall income:
- Add your numbers up. The total here is 2,378.
- Add up the sum total for the top fifth. the total is 580 + 610 = 1,190
- Divide Step 2 by Step 1: 1,190 / 2,378 = 0.5004.
- Multiply Step 3 by 100: 0.5004 * 100 = 50.04%
The top fifth accounts for over 50% of incomes.
Quintile Use in Economics and Health Statistics
Quintiles are often used in reference to economic differences by politicians and policy-makers; for instance, the top 20% of wage earners might be called the fifth quintile.
Health organizations sometimes use quintiles to show distributions of health statuses. For example, the following image from the World Health Organization shows tobacco use, by wealth quintile, for 13 countries:
Reybnolds, A. (2006). Income and Wealth. Greenwood Publishing Group.
World Health Organization (2016). World Health Statistics 2016: Monitoring Health for the SDGs Sustainable Development Goals. Available on Google Books.