There are three measures of spread used in National 5 Lifeskills Maths.
range
semi-interquartile range
standard deviation
The range includes extreme values.
The semi-interquartile range ignores extreme values.
Look at this set of data \(1,5,7,8,9,12,13,15,17,18,35\)
The range is \(35-1=34\)
The semi-interquartile range is \(\frac{1}{2}\times (17-7) = 5\). In cases like these it's often preferable to use the semi-interquartile range when you're comparing data.