SNOMED CT® Release Format 1 (RF1) Guide - ihtsdo

Figure 25: Alternative expressions that mean "red steel pedal bicycle"
Two rules limit the range of equivalent expressions:
Rule 1: It is not possible to transform a primitive concept into a post-coordinated expression.
Structure and Content Guide | 59
• A primitive concept has a facet that is not represented by its defining characteristics and therefore any
attempt to represent it in a post-coordinated form results in a loss of information.
This is illustrated by consideration of the definitions of the concept "bicycle" in Table 14. The definition stated
in the table is as follows:
| is a | " = "Transport device", | Used as | = | Transport |, | Moves on | = | 2 wheels |, | Origin | = | Man made
|
This definition would also apply to a horse-drawn cart or a trailer. Therefore the concept "bicycle" must be
regarded as primitive. Recognizing this fact means that some of the apparently equivalent expressions in
Figure 25 cannot be computed as equivalent. Unless the focus concept is a subtype of "bicycle" it is not
possible to compute that it is a kind of bicycle. This means that to create an equivalent expression it would
be necessary to add | is a | = "Bicycle". This is shown in Figure 26.
Examining these definitions, it is apparent that the characteristics shown in gray are redundant because they
are part of the definition of "bicycle."
As a consequence of this rule, primitive concepts create the limits on the ability to transform an expression
to a more post-coodinated form. An expression can be normalized until all the concepts referred to by the
expression are primitive. An expression in which all the referenced concept are primitive is referred to as the
normal form.
© 2002-2012 International Health Terminology Standards Development Organisation CVR #: 30363434