openEHR data types currently do not have a quantitative type representing the idea of a
pure ratio, e.g. for titers, 1:128 (integral), Na:K concentration ratio (unitary denominator),
albumin:creatinine ratio, and % e.g. red cell distribution width (RDW) which
is the width of a distribution of RBC widths.
The characteristics of this data type are:
units of numerator and denominator are the same, or else don't matter
values can be real or integral
should be expressible as a percentage if required