The fundamentals of quantitative measurement
- *School of Nursing, Faculty of Health Sciences, McMaster University, Hamilton, Ontario, Canada
- 2Centre for Evidence Based Nursing, Department of Health Studies, University of York, York, UK
The main purpose of the EBN Notebook is to equip readers with the necessary skills to critically appraise primary research studies and to provide a more detailed description of some of the methodological issues that arise in the papers we abstract. In the July 1999 issue of Evidence-Based Nursing, the EBN Notebook explored the concept of sampling.1 In this issue we will provide a basic introduction to quantitative measurement of health outcomes, which may be assessed in studies of treatment, causation, prognosis, diagnosis, and in economic evaluations. Examples of health related outcomes are blood pressure, quality of life, patient satisfaction, and costs.
Health can be measured in many different ways; the various aspects of health that can be measured are referred to as variables.2 For example, in the treatment study by Dunn et al in this issue of Evidence-Based Nursing (p 117), the interventions (known as the independent variables) were lifestyle and structured exercise programmes and the outcomes (known as the dependent variables) were physical activity and cardiorespiratory fitness. In a treatment study, the independent variables are those that are under the control of the investigator, and the dependent variables are the outcomes that may be influenced by the independent variable. In a causation study, the investigator relies on natural variation between both variables and looks for a relation between the 2 variables. For example, when determining whether smoking causes lung cancer, smoking is the independent variable and lung cancer is the dependent variable. In the abstracts included in Evidence-Based Nursing, the independent variables are identified under the “intervention” section for treatment studies and under the “assessment of risk factors” section for causation studies. The dependent variables are identified under the “main outcome measures” section.
Types of variables
Variables can be classified as nominal, ordinal, interval, or ratio variables. …