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* The second quartile (''Q''<sub>2</sub>) is the median of a data set; thus 50% of the data lies below this point.
* The third quartile (''Q''<sub>3</sub>) is the middle value between the median and the highest value ([[Sample maximum and minimum|maximum]]) of the data set. It is known as the ''upper'' quartile, as 75% of the data lies below this point.<ref name=":0">{{Cite book|title=A modern introduction to probability and statistics: understanding why and how|url=https://archive.org/details/modernintroducti0000unse_h6a1|url-access=limited|date=2005|publisher=Springer|others=Dekking, Michel, 1946–|isbn=978-1-85233-896-1|location=London|pages=[https://archive.org/details/modernintroducti0000unse_h6a1/page/236/ 236-238]|oclc=262680588}}</ref>
Along with the minimum and maximum of the data (which are also quartiles), the three quartiles described above provide a [[five-number summary]] of the data. This summary is important in statistics because it provides information about both the [[Mean (Statistics)|center]] and the [[Statistical dispersion|spread]] of the data. Knowing the lower and upper quartile provides information on how big the spread is and if the dataset is [[Skewness|skewed]] toward one side. Since quartiles divide the number of data points evenly, the [[Range (statistics)|range]] is usually not the same between quartiles (i.e. usually ''Q''<sub>3</sub>-''Q''<sub>2</sub>
== Definitions ==
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