The interquartile range is commonly used, i.e. the distance between
the 25th and 75th percentiles (Q3 - Q1). N.B. By definition, this
contains 50% of the datapoints in a normally-distributed dataset.
The semi-interquartile range, i.e. (Q3 - Q1)/2, covers 25% of the
datapoints and is even less subject to variation due to scatter.