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The median of the data has been referred as the second quartile Q ₂ .Also, the first quartile Q ₁ is the median of the first half of the data, and the third quartile Q ₃ is the median of the second half of the data.  Quartile deviation is the dispersion in the middle of the data.

The semi interquartile range (SIR) (also called the quartile deviation) is a measure of spread. It tells you something about how data is dispersed around a central point (usually the mean).

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