ABSTRACT
Researchers measure a range of variables. Those interested in laboratory science may measure cell replication or decay and the impact of a new drug upon this process. For those interested in clinical medicine, blood pressure, weight, or glucose metabolism may be what they measure whilst epidemiologists focus on illness prevalence and incidence. Such research involves questions such as ‘How many cells are there’?, ‘What is their rate of decay’?, ‘What is an individual’s level of insulin production’?, or ‘How many new cases of cancer are there each year’? These questions require a simple process of measurement as the cells and cases are counted and the numbers shown on weighing scales or blood pressure machines are recorded. The machines may need to be calibrated but no one asks the questions ‘How do you know’?, ‘Are you sure’? or ‘Says who’? What it is, is what is measured and measurement is simple (or so they tell us!). For many research studies, however, measurement is much more complex. This chapter will explore different types of measurement, theories of measurement, and how we can check the usefulness of any measurement tool focusing on conceptualisation, operationalisation, reliability, and validity. These different issues will then be illustrated in terms of how we can measure one variable: health status.
