There is no “other” category: When to avoid “miscellaneous”

It is often tempting when describing a series of subclasses that you know are not exhaustive to add a class labelled as “Other” or “Miscellaneous”. This is generally seen to be a bad idea for a series of reasons.

Take a class such as “Molecular function” in the Gene Ontology (GO) (and it must be noted that the GO is not guilty of this ontological crime), then we might add a series of subclasses describing, for instance, enzyme functions, motility functions, binding functions, and so on.  The temptation is to have a class called either “Miscellaneous”, “Function unknown” or “other”.

The semantics of many languages used for representing ontologies can easily help us out. If the function we want is not in the ontology underneath “molecular function” or we do not know the function, all we have to do is label our gene product as having “Molecular function”. Our description says the gene product has a function, but does not say what it is. The semantics of this statement are as clear as saying a gene product has an unknown function.

Another problem with “Unknown”, “Other” or “Miscellaneous” is that the types of object in that category keep changing. As the unknown becomes known, then subsets of that category keep disappearing. So, we have no real idea of what “unknown” means.

Finally, from the point of view of realism, it can also be argued that “unknown function” does not exist anyway. Gene products may not have “unknown function” but functions which we do not yet know – which is a different thing altogether.

[Article authored by Robert Stevens and Duncan Hull]