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Nominal or categorical data are data that one can name and put into categories. They are not measured but simply counted. They often consist of unordered ‘either‐or’ type observations that have two categories and are often know as binary. For example: dead or alive; male or female; cured or not cured; pregnant or not pregnant. In Table 2.1 gender is a binary variable. However, categorical data often can have more than two categories, for example: blood group O, A, B, AB, country of origin, ethnic group or social class. The methods of presentation of nominal data are limited in scope. Thus Table 2.1 gives the number and percentage of people treated at each of the seven centres in each of the two randomised groups. Categorical data is sometimes referred to as ‘qualitative’, to distinguish it from ‘quantitative’ which we will discuss later. However, there is a whole area of methodology called ‘qualitative research’ and so to avoid confusion we will not us this term.