Quartiles are values that divide a sample of data into four equal parts. With them you can quickly evaluate a data set's spread and central tendency, which are important first steps in understanding your data.
|1st quartile (Q1)||25% of the data are less than or equal to this value.|
|2nd quartile (Q2)||The median. 50% of the data are less than or equal to this value.|
|3rd quartile (Q3)||75% of the data are less than or equal to this value.|
|Interquartile range||The distance between the 1st and 3rd quartiles (Q3-Q1); thus, it spans the middle 50% of the data.|
Quartiles are calculated values, not observations in the data. It is often necessary to interpolate between two observations to calculate a quartile accurately.
Because they are not affected by extreme observations, the median and interquartile range are a better measure of central tendency and spread for highly skewed data than are the mean and standard deviation.