What Is Interquartile Range (IQR)?

The Interquartile Range (IQR) is a measure of statistical dispersion, which is the range between the first quartile (Q1) and the third quartile (Q3). It represents the middle 50% of the data in a dataset.

The Interquartile Range (IQR) is a measure of statistical dispersion, which quantifies how spread out the data points in a dataset are. Specifically, the IQR is the range between the first quartile (Q1) and the third quartile (Q3), thus capturing the middle 50% of data points. It is considered a robust measure of variability because it is divided by the median, making it less sensitive to outliers.

Formula and Calculation

The IQR is calculated using the following formula:

$$ \text{IQR} = Q_3 - Q_1 $$

Where:

• \( Q_1 \) is the first quartile (25th percentile)
• \( Q_3 \) is the third quartile (75th percentile)

The steps to calculate the IQR are as follows:

• Arrange Data: Sort the data in ascending order.
• Calculate Quartiles: Find the values of \( Q_1 \) and \( Q_3 \).
• Compute IQR: Subtract \( Q_1 \) from \( Q_3 \).

Example

Consider the dataset: 4, 7, 8, 9, 10, 15, 21.

• Arrange Data: The data is already sorted.
• Calculate Quartiles:
  • \( Q_1 \) is the median of the first half (4, 7, 8) which is 7.
  • \( Q_3 \) is the median of the second half (10, 15, 21) which is 15.
• Compute IQR:
  • \( \text{IQR} = 15 - 7 = 8 \)

Importance and Applications

The IQR is crucial in fields like statistics, economics, and psychology for the following reasons:

• Robustness to Outliers: Unlike range, the IQR is not affected by extremely high or low values.
• Identifying Outliers: Data points that lie outside of 1.5 * IQR from the quartiles are potential outliers.
• Comparison of Distributions: The IQR can compare the spread between different datasets, providing insights into variability.

Comparisons with Other Measures of Dispersion

Range

• Definition: The difference between the maximum and minimum values.
• Sensitivity: Highly sensitive to outliers.
• Usage: Useful for small datasets but not robust.

Standard Deviation

• Definition: Measures the amount of variation or dispersion in a dataset.
• Calculation: Takes into account every data point.
• Usage: Commonly used but can be influenced by outliers.

Related Terms

• Quartile: Values that divide a dataset into four equal parts.
• Median: The middle value in a dataset.
• Percentile: Indicates the relative standing of a value in a dataset.

FAQs

What is an outlier and how is it detected using the IQR?

An outlier is a data point that deviates significantly from the other observations. It can be detected using the IQR by identifying points that fall below \( Q_1 - 1.5 \times \text{IQR} \) or above \( Q_3 + 1.5 \times \text{IQR} \).

How is the IQR different from the range?

The range measures the spread between the minimum and maximum values and is sensitive to outliers. The IQR measures the spread of the middle 50% of values, making it less sensitive to outliers.

References

1. Weisstein, Eric W. “Interquartile Range.” From MathWorld–A Wolfram Web Resource. https://mathworld.wolfram.com/InterquartileRange.html
2. “Descriptive Statistics.” The University of Texas at Austin. https://stats.libretexts.org/Bookshelves/Descriptive_Statistics_6e
3. “Statistics: Unlocking the Power of Data.” Lock, Lock, Lock, Lock, and Lock.

Summary

The Interquartile Range (IQR) is a robust measure of statistical dispersion that captures the range between the first and third quartiles of a dataset. By focusing on the middle 50% of the data, it provides a clear view of data variability while minimizing the influence of outliers. The IQR is essential for both simple descriptive statistics and more complex data analyses, enabling a deeper understanding of data spread and the identification of outliers.