Know your quartiles

Paul Wellings, vice-chancellor of Lancaster University,

claims that his university "is now in the top quartile of

UK institutions" (Letters,

July 20).

No university is that, since a quartile is not a segment of a distribution but a boundary.

There are three quartiles: One quarter of a rank- ordered group is below the lower quartile and one quarter is above the upper quartile.

The middle quartile splits the distribution in two and is called the median.

Stephen Senn

Professor of statistics

Glasgow University