Range and Interquartile Range

The variates of a data are real numbers (usually integers). So, thay are scattered over a part of the number line. An investigator will always like to know the nature of the scattering of the variates. The arithmetic numbers associated with distributions to show the nature of scattering are known as measures of dispersion. Simplest of them are:

(i) Range

(ii) Interquartile Range.

Range: The difference of the greatest variate and the smallest variate in a distribution is called the range of the distribution.

Interquartile Range: The interquartile range of a distribution is Q₃ - Q₁, where Q₁ = lower quartile and Q₃ = upper quartile.

$\frac{1}{2}$(Q₃ - Q₁) is known as semi-interquartile range.

Solved Examples on Range and Interquartile Range:

1. The following data represent the number of books issued by a library on 12 different days.

96, 180, 98, 75, 270, 80, 102, 100, 94, 75, 200, 610.

Find the (i) interquartile range, (ii) semi-interquartile range and (iii) range.

Solution:

Write the data in ascending order, we have

75, 75, 80, 94, 96, 98, 100, 102, 180, 200, 270, 610.

Here, N = 12.

So, $\frac{N}{4}$ = $\frac{12}{4}$ = 3, which is an integer.

Therefore, the mean of the 3rd and 4th variates is Q₁ = $\frac{80 + 94}{2}$ = $\frac{174}{2}$ = 87.

So, $\frac{3N}{4}$ = $\frac{3 × 12}{4}$

                                = $\frac{36}{4}$

                                = 9, i.e., $\frac{3N}{4}$ is an integer.

Therefore, the mean of the 9^th and 10^th variates is Q₃ (the upper quartile).

Therefore, Q₃ = $\frac{180 + 200}{2}$

                     = $\frac{380}{2}$

                     = 190.

(i) Interquartile Range = Q₃ - Q₁ = 190 - 87 = 103

(ii) Semi-interquartile Range = $\frac{1}{2}$(Q₃ - Q₁)

                                          = $\frac{1}{2}$(190 - 87)

                                          = $\frac{103}{2}$

                                          = 51.5.

(iii) Range = Highest Variate -  Lowest Variate 

                = 610 - 75

                = 535.

2. Marks obtained by 70 students in an examination are given below.

Find the interquartile range.

Solution:

Arrange the data in ascending order, the cumulative-frequency table is constructed as below.

Here, $\frac{N}{4}$ = $\frac{70}{4}$ = $\frac{35}{2}$ = 17.5.

Cumulative frequency just greater than 17.5 is 18.

The variate whose cumulative frequency is 18, is 35. 

So, Q₁ = 35.

Again, $\frac{3N}{4}$ = $\frac{3 × 70}{4}$ = $\frac{105}{4}$ = 52.5.

Cumulative frequency just greater than 52.5 is 61.

The variate whose cumulative frequency is 61, is 65.

Therefore, Q₃ = 65.

Thus, Interquartile Range = Q₃ - Q₁ = 65 - 35 = 30.

9th Grade Math

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