The points that divide the distribution in to four equal parts are called quartiles. If we take the difference between the third quartile and the first quartile, it gives us a value called inter quartile range. It is equal to
- Q3 – Q1. Half of this is the Semi-Inter Quartile Range or quartile deviation which is (Q3 – Q1)/2 .
- Quartile deviation is also called the semi-inter quartile range.
Formula to find the quartile deviations:
First quartile(Q1) = n+1/4 th score
Median(Q2) = n+1/2 th score
Third quartile(Q3) = 3(n+1)/4 th score
Quartile deviation = Q3 – Q1/2 th score
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