1 Where do data come from?

Data can take many forms. Sometimes we are dealing with simple numbers, such as blood pressure measurements or age. Some data may consist of categories, such as country of origin or presence/absence of a characteristic of interest. Modern data can be much more sophisticated, arising in the form of images, temperature curves, three-dimensional surfaces or networks. This chapter focuses on relatively simple data forms but uses these to discuss some important principles of how data are collected and what they are able to tell us.