The difference 
,
where 
and 
are hinges. It is the same as
the interquartile range for 
, 9, 13, ... points.
H-Spread
See also
Hinge, Interquartile Range, StepExplore with Wolfram|Alpha
References
Tukey, J. W. Exploratory Data Analysis. Reading, MA: Addison-Wesley, p. 44, 1977.Referenced on Wolfram|Alpha
H-SpreadCite this as:
Weisstein, Eric W. "H-Spread." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/H-Spread.html