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CHAPTER 5 Descriptive Studies: Quantitative

Broadly speaking, quantitative research may be either observational or experimental. In the first, the researcher actively observes patients by doing things like asking questions and taking samples, but does not experiment with the patient's treatment or care. In a typical experimental study, in contrast, the researcher intervenes to ensure that some or all of a group of people receive a treatment, service, or experience.

It can be helpful to divide observational studies into two groups according to their complexity (Figure 5.1). On the one hand, descriptive observational studies ask questions like: ‘What are the clinical or biochemical characteristics of people who have rheumatoid arthritis?', ‘How common a condition is asthma?', or ‘How disabled do people become over a decade of follow‐up after a diagnosis of multiple sclerosis?' On the other hand, some observational studies compare groups to try to answer more complex questions: ‘Do people who smoke get more heart disease than those who don't smoke?' or ‘Are women who experienced venous thrombosis more likely to have been taking the oral contraceptive pill than are women who didn't sustain a thrombosis?' Studies that ask these kinds of non‐experimental (observational) questions, but which involve comparisons, are often described as analytic.

Figure 5.1 Types of research study design.

Analytic observational studies are dealt with in Chapter 6 and experimental (intervention) studies in Chapter 7. The remainder of this chapter tackles the simplest forms of quantitative observation – descriptive studies. We find it useful to subdivide descriptive studies into four types:

 Case reports

 Case series

 Cross‐sectional studies (simple cross‐sectional studies determining, for example, how common (prevalent) a condition is; more complex cross‐sectional studies involving comparisons are dealt with under analytic research in Chapter 6)

 Longitudinal studies

CASE REPORTS

Some would say that case reports are scarcely research at all. They usually take the form of an unusual clinical case that illustrates something about the cause, or the outcome of the person described that the author hopes will intrigue you. Perhaps the author's care over detail – eliciting symptoms, possible precipitants, and treatments offered – takes the case report out of the ordinary clinical arena and justifies the title of research. Research journal editors vary in their views – some publish such reports and others do not.

CASE SERIES

A respectable form of research is a description of clinical findings seen in a succession of patients who seem to display a similar condition: the case series. Something unexpected has turned up – more cases than usual of a rare disorder perhaps, or an apparent excess of some clinical sign – hence the motive for writing‐up the series of cases. For example, Figure 5.2 shows how clinicians used a case series to point to dangers of playing around on bouncy castles.

Figure 5.2 Extract from case series of bouncy castle injuries.

Source: From Singer and Freedman (1992), © 1992, BMJ Publishing Group Ltd.

Sometimes the author of a case series notices some common feature that the cases share and speculates that this factor might help to explain the condition. A famous example of such studies includes the early descriptions of the birth abnormalities that became linked with the drug thalidomide.

CROSS‐SECTIONAL STUDIES

Unlike the case series, which usually reports an unexpected clinical encounter, the researchers of a cross‐sectional study deliberately set out to assemble a group of study subjects – often patients in current or recent contact with part of the health service – and describe the presence and absence of various clinical features. For example, responding to the case series of bouncy castle injuries (Figure 5.2), another researcher extracted from a national survey of leisure accidents the clinical and circumstantial details of 105 bouncy castle injuries (Figure 5.3) – providing us with a more representative picture. This new, premeditated study of 105 cases is a fairly typical cross‐sectional study. It seems to show that, when the matter is examined using a suitably selected sample, there is no convincing evidence that bouncy castle injuries cause a specific fracture of the elbow region but, instead, lead to all manner of soft tissue, joint, and bony injuries – anywhere on the body.

Figure 5.3 Extract from cross‐sectional study about bouncy castle injuries.

Source: From Levene (1992), © 1992, BMJ Publishing Group Ltd.

A particular type of cross‐sectional, single‐group study is one in which incidence or prevalence of a condition is determined. A prevalence study determines how many cases of a condition or disease there are in a given population, or it might establish the frequency of a clinical finding in a study sample. For example, researchers might use hospital and pharmacy records to establish how many people have insulin‐receiving diabetes in a defined hospital catchment area. Then again, another study might estimate the proportion of older people in residential facilities whose ears are occluded by earwax (Figure 5.4).

Figure 5.4 Prevalence of a clinical feature, determined in a cross‐sectional study.

Source: From Culbertson et al. (2004), © 2004, Elsevier.

Incidence studies are rather similar but refine the above kind of study in two ways: the incidence of a condition is the number of new cases arising in a defined population over a defined time. Figure 5.5 describes such a study – to determine in a defined area of 22 adjoining electoral wards in South London, the number of first‐in‐a‐lifetime strokes, according to ethnic origin. The researchers established the incidence (sometimes called inception rate) of stroke in each of the ethnic groups under scrutiny – per 1000 population per year. Strictly speaking it is this incorporation of time, as well as the proportion of cases, that makes incidence a rate, while prevalence is merely a proportion.

Figure 5.5 Extract from a cross‐sectional (incidence) study about frequency of stroke.

Source: From Wolfe et al. (2002), © 2002, BMJ Publishing Group Ltd.

Figure 5.5 shows how the data were collected from hospitals and the community. Some incidence research data, as in this case, are derived from case‐registers – many of which have been set up with research and clinical service in mind, routinely recording data useful for both purposes. In other cross‐sectional studies the researchers undertake a survey of the study sample, where their survey – whether by interview, or by electronic or paper self‐report – has been set up specifically for the research project. For example, researchers might ask nurses who visit patients in their homes to describe, by filling in a paper questionnaire, how their patients are using prescribed medicines (Figure 5.6).

Figure 5.6 Use of a survey method in a cross‐sectional study.

Source: From Ellenbecker et al. (2004), © 2004, Elsevier.

LONGITUDINAL STUDIES

When researchers study a group of subjects over time in a longitudinal study, there is more research work to be done than in a cross‐sectional study; subjects must be followed up one or more times to determine their prognosis or outcome.

The kinds of observational studies we've seen above are among the simplest form of clinical research. In the next chapter, more complex observational studies are described – quantitative analytic studies. What they have in common (and in this way they differ from the above study types) is that they generally involve comparison of two or more groups of people and often attempt to infer something about cause of symptoms or conditions.