In Statistics, the interquartile range is the smallest of all the measures of dispersion. It is the difference between the two extreme conclusions of the distribution. In other words, the range is the difference between the maximum and the minimum observation of the distribution
It is defined by
Range = X_{max} – X_{min}
Where X_{max} is the largest observation and X_{min} is the smallest observation of the variable values.
Interquartile Range Definition
The interquartile range defines the difference between the third and the first quartile. Quartiles are the partitional values that divide the whole series into 4 equal parts. So, there are 3 quartiles. First Quartile is denoted by Q_{1 }known as the lower quartile, the second Quartile is denoted by Q_{2} and the third Quartile is denoted by Q_{3} known as the upper quartile. Therefore, the interquartile range is equal to the upper quartile minus lower quartile.
Interquartile Range Formula
The difference between the upper and lower quartile is known as the interquartile range. The formula for the interquartile range is given below
Interquartile range = Upper Quartile – Lower Quartile = Q_{3} – Q_{1}
where Q_{1} is the first quartile and Q_{3} is the third quartile of the series
Semi Interquartile Range
The semiinterquartile range is defined as the measures of dispersion. Semi interquartile range also is defined as half of the interquartile range. It is computed as one half the difference between the 75th percentile (Q_{3}) and the 25th percentile (Q_{1}). The semiinterquartile range is onehalf of the difference between the first and third quartiles. The Formula for Semi Interquartile Range is
Semi Interquartile Range = (Q_{3}– Q_{1}) / 2
Median and Interquartile Range
The median is the middle value of the distribution of the given data. The interquartile range (IQR) is the range of values that resides in the middle of the scores. When a distribution is skewed, and the median is used instead of the mean to show a central tendency, the appropriate measure of variability is the Interquartile range.
Q_{1 }– Lower Quartile Part
Q_{2} – Median
Q_{3} – Upper Quartile Part
It is a measure of dispersion based on the lower and upper quartile. Quartile deviation is obtained from interquartile range on dividing by 2, hence also known as semi interquartile range.
How to Calculate the Interquartile Range?
The procedure to calculate the interquartile range is given as follows:
 Arrange the given set of numbers into increasing or decreasing order.
 Then count the given values. If it is odd, then the centre value is median otherwise obtain the mean value for two centre values. This is known as Q_{2 }value
 Median equally cuts the given values into two equal parts. They are described as Q_{1} and Q_{3} part.
 From Q_{1 }values, we have to find one median value.
 From Q_{3} values, we have to find one median value.
 Finally, we can subtract the median values of Q_{1} and Q_{3}.
 The resulting value is the interquartile range.
Interquartile Range Example
Question: Determine the interquartile range value for the first ten prime numbers.
Solution:
Given: The first ten prime numbers are:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29
This is already in increasing order.
Here the number of values = 10
10 is an even number. Therefore, the median is mean of 11 and 13
That is Q_{2} = 12.
Now we have to get two parts Q_{1 }and Q_{3}.
Q_{1} part : 2, 3, 5,7,11
Here the number of values = 5
5 is an odd number. Therefore, the centre value is 5, that is Q_{1}= 5
Q_{3} part : 13, 17, 19, 23, 29
Here the number of values = 5
5 is an odd number. Therefore, the centre value is 19, that is Q_{3}= 19
The subtraction of Q1 and Q_{3 }value is 19 – 5 = 11
Therefore, 11 is the interquartile range value.
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