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Types of data


At the end of the lecture and having completed the exercises students should be able to:

The methods of description and analysis we apply to data sets are dependent on the type of variable we are considering.

Categorical data

Variables of this type have values that can be distinguished from each another.


Nominal variables have values that can be distinguished from each another. An example might be the variable 'Sex' which can have values 'Male' and 'Female'


Ordinal variables not only has values that can be put into some sort of meaningful order. If we asked patients how much pain they felt and gave them the options of 'None', 'A bit', 'Quite a lot', 'More than I can bear' then we could put their responses into a meaningful order. We could say that 'Pain' was an ordinal variable.

Metric data

Metric variables can not only have their values put into a meaningful order but can be measured on an 'equal interval' scale. For a metric variable the difference between 1 and 2 is the same as the difference between 2 and 3; this is not the case for an ordinal variable.


Discrete variables are countable variables such as the number of teeth a patient has.


Continuous variables can be measured to any degree of accuracy, in theory. Blood pressure is a continuous variable.


It is important to be able to distinguish different types of data from one another as we use different techniques to describe and analyses the different types. The distinction between categorical and metric data is more important the the subdivisions of those types. There are other distinctions which are not so important for us at this point. Perhaps the most common of these is the distinction between ratio and interval subdivisions of continuous metric data.

Another type

Ratio class data is continuous data which has a true zero. For example weight in kilos is a ratio class variable: someone who weighs 80kg is twice as heavy as someone who weighs 40kg. (I.e., the values can be expressed properly as a ratio.) Temperature measured in degrees Celcius, however, is merely an interval class variable. Something that is at 20°C is not twice as hot as something at 10°C. The two values cannot be properly expressed as a ratio as the zero-point on the Celcius scale is arbitrarliy chosen (at the freezing point of water). Do not worry too much about this distinction as it is not necessary for this course.