Child and Adolescent Psychiatry

Classification 25

and 2.2). Factor analyses are used to identify dimensions while cluster
analyses identify categories. Whereas factor analyses classify attributes of
an individual, cluster analyses classify the individuals themselves.

Box 2.2A do-it-yourself cluster analysis

Look at the next list of different animals and divide them into groups:

Tortoise

Duck-billed platypus

Cat

Snail

Dolphin

Crocodile

Mouse

Giant squid

As you attempted to do so, you will probably have identified some of the key

features and limitations of a cluster analysis. First, you may have noticed that

how you grouped the animals depended on which features you concentrated on.

If you had focused on measures of size and habitat, you might have grouped

dolphins, crocodiles and giant squid together as large aquatic animals, and snails

and mice together as small terrestrial animals. If, however, you concentrated

on morphological and physiological measures, you would have generated a

more typically zoological taxonomy, for example, generating a mollusc grouping

comprised of snails and giant squid. A second notable feature of cluster analysis is

that the method does not tell you how many groups to identify. For instance, you

could have gone for a ‘two group’ solution (for example, mollusc v. vertebrate)

or a ‘three group’ solution (for example, mollusc v. reptile v. mammal). You have

to decide for yourself what degree of lumping or splitting is most appropriate

(which depends on what use you plan to make of the classification). Finally, the

case of the duck-billed platypus, with its mixture of reptilian and mammalian

features, is a reminder that some individuals fall midway between neighbouring

categories – it is somewhat arbitrary whether they are assigned to one of the

neighbouring categories or to a category of their own.

Pervasive or situational?
For hyperactivity problems, and perhaps for other types of problem too,
diagnostic schemes increasingly emphasise the distinction between perva-
sive and situational disorders. Pervasive disorders are evident in a wide
variety of everyday settings (for example, at home and at school), whereas
situational disorders are only evident in a restricted range of settings
(for example, at home but not at school). Pervasiveness suggests that
constitutional factors are paramount, while situational specificity suggests
that it is more important to establish what is special about that particular
environment (or that particular informant).
Unfortunately, the term ‘pervasive’ is used in two very different ways
in classifications of child and adolescent psychiatric disorders. Pervasive