Quartile

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In descriptive statistics, a quartile is any of the three values which divide the sorted data set into four equal parts, so that each part represents 1/4th of the sampled population.

Contents 1 Definitions 2 Computing methods 2.1 Example 3 See also 4 External links

[edit] Definitions

• first quartile (designated Q₁) = lower quartile = cuts off lowest 25% of data = 25th percentile
• second quartile (designated Q₂) = median = cuts data set in half = 50th percentile
• third quartile (designated Q₃) = upper quartile = cuts off highest 25% of data, or lowest 75% = 75th percentile

The difference between the upper and lower quartiles is called the interquartile range.

[edit] Computing methods

There is no universal agreement on choosing the quartile values.[1]

[edit] Example

One possible rule (employed by the TI-83 calculator boxplot and 1-Var Stats functions) is as follows:

1. Use the median to divide the ordered data set into two halves. Do not include the median into the halves.
2. The lower quartile value is the median of the lower half of the data. The upper quartile value is the median of the upper half of the data.

The examples below assume this rule. Another possible rule would be to include the median in the halves when calculating the quartiles. This would give significantly different answers to the examples.

Example 1
Data Set: 6, 47, 49, 15, 42, 41, 7, 39, 43, 40, 36
Ordered Data Set: 6, 7, 15, 36, 39, 40, 41, 42, 43, 47, 49

Example 2
Ordered Data Set: 7, 15, 36, 39, 40, 41

Example 3
Ordered Data Set: 1 2 3 4

[edit] See also

• Range
• Quantile
• Quarter
• Summary statistics

[edit] External links

• Quartile - from MathWorld Includes references and compares various methods to compute quartiles
• Quartiles - From MathForum.org
• Quartiles - An example how to calculate it