This is half of
the distance from the lower quartile to the upper quartile.

In other words, the semi interquartile range is the interquartile
range divided by 2. The Interquartile Range is the half difference between
the 75th percentile(Q3) and 25th percentile(Q1). It is a robust measure of variability.

Therefore the formula of semi-interquartile range is (Q3 - Q1) / 2.