Before we can examine normal reactions to trauma, it is necessary to come to some understanding of what is and is not normal. This may seem pedantic or simplistic, but it is not as simple as it seems.

The Oxford English Dictionary contains a large number of definitions of the word normal. Some are specific to physics or chemistry and are not relevant. Others include:

• the usual state or condition

• constituting, conforming to, not deviating or differing from, the common type or standard; regular, usual

• a normal variety of anything; that which, or a person who, is healthy and is not impaired in any way.

It is a little depressing to see that even The Oxford English Dictionary uses the word normal in the definition of the word normal. It seems that there are two relevant definitions of normal. One refers to the frequency of occurrence of the matter under study, that it is or is not the usual or common state. The other refers to an absence of disease. The two are not synonymous. For example, acne is probably statistically normal in adolescence but is not a healthy-normal. More dramatically, the statistically normal response to ingestion of an inoculum of Vibrio cholerae is probably to develop cholera.

It is fairly easy to define a concept of statistical normality. For example, the modal response might be considered to be the statistical norm.