Interquartile Range

Measurement data can be divided into quartiles.  In other words, we can talk about the lowest 25% of cases, the next lowest 25%, the second highest 25%, and the highest 25%.  The division between the lowest 25% and the upper 75% is called the first quartile (Q1).  The division between the lower 50% and the upper 50% is called the median.  It could also be called the second quartile, but we give it a special name just because we pay special attention to the middle.  The division between the lower 75% and the upper 25% is called the third quartile (Q3).  The difference between Q3 and Q1 is called the interquartile range (IQR).  The IQR measures how spread out the middle 50% of cases are.  The larger the IQR, the more variety there is in the data.  Since the IQR is based on the middle 50% of cases, it is resistant to outliers.

SPSS instructions for getting quartiles

SPSS instructions for getting the IQR