Question
Find the lower quartile, the upper quartile, the interquartile range and the semi-interquartile range for the following frequency distributions: 
Shoe size  5 6 7 8 9 10 11
Frequency 8 1 7 14 11 5 4

Answer

Shoe size  Frequency (f) Cumulative frequency
5 8 8
6 1 9
7 7 16
8 14 30
9 11 41
10 5 46
11 4 50
No. of terms $=50$
Lower Quartile $\left( Q _1\right)=\frac{n}{4}=\frac{50}{4}=12.5^{\text {th }}$ term $=7$
Upper Quartile $\left( Q _3\right)=\frac{n \times 3}{4}=\frac{50 \times 3}{4}=37.5^{\text {th }}$ term $=9$
Interquartile range $=Q_3-Q_1=9-7=2$
Semi-interquartile range $=\frac{Q_3-Q_1}{2}=\frac{9-7}{2}=1$
Hence, Lower quartile $=7$, upper quartile $=9$, interquartile range $=2$, semi-interquartile range $=1$

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