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Series on the Evaluation of Scientific Publications
R E V I E W A RT I C L E
Study Design in Medical Research Part 2 of a Series on the Evaluation of Scientific Publications
Bernd Röhrig, Jean-Baptist du Prel, Maria Blettner
SUMMARY Background: The scientific value and informativeness of a medical study are determined to a major extent by the study design. Errors in study design cannot be corrected afterwards. Various aspects of study design are discussed in this article.
Methods: Six essential considerations in the planning and evaluation of medical research studies are presented and discussed in the light of selected scientific articles from the international literature as well as the authors’ own scientific expertise with
regard to study design.
Results: The six main considerations for study design are the question to be answered, the study population, the unit of analysis, the type of study, the measuring technique, and the calculation of sample size.
Conclusions: This article is intended to give the reader guidance in evaluating the design of studies in medical research. This should enable the reader to categorize medical studies better and to assess their scientific quality more accurately.
Dtsch Arztebl Int 2009; 106(11): 184–9 DOI: 10.3238/arztebl.2009.0184
Key words: study design, quality, study, study type, measuring technique
Institut für Medizinische Biometrie, Epidemiologie und Informatik (IMBEI), Johannes Gutenberg-Universität Mainz: Dr. rer. nat. Röhrig, Prof. Dr. rer. nat. Maria Blettner
Zentrum Präventive Pädiatrie, Zentrum für Kinder- und Jugendmedizin, Johannes Gutenberg-Universität Mainz: Dr. med. du Prel, M.P.H
Deutsches Ärzteblatt International⏐⏐Dtsch Arztebl Int 2009; 106(11): 184–9 185
M E D I C I N E
and possibly that new hypotheses are generated. The following questions are important: Why? Who? What? How? When? Where? How many? The question to be answered also implies the target group and should therefore be very precisely formulated. For
ex- ample, the question should not be “What is the quality of life?”, but must specify the group of patients (e.g. age), the area (e.g. Germany), the disease (e.g. mam- mary carcinoma), the condition (e.g. tumor stage 3), perhaps also the intervention (e.g. after
surgery), and what endpoint (in this case, quality of life) is to be de- termined with which method (e.g. the EORTC QLQ- C30 questionnaire) at what point in time. Scientific questions are often not only purely descriptive, but also include comparisons, for
example, between two groups, or before and after the intervention. For example, it may be interesting to compare the quality of life of breast cancer patients with women of the same age without cancer.
The research worker specifies the question to be an- swered, and whether the study is to be evaluated in a descriptive, exploratory or confirmatory manner. Whereas in a descriptive study the units of analysis are to be described by the recorded variables (e.g.
blood parameters or diagnosis), the aim in an explor- atory analysis is to recognize connections between variables, to evaluate these and to formulate new hypotheses. On the other hand, confirmatory analyses are planned to provide statistical proofs by testing specified study hypotheses.
The question to be answered also determines the type and extent of the data to be recorded. This specifies which data are to be recorded at which point in time. In this case, less is often more. Data irrelevant to the question(s) to be answered should not be
collected for the moment. If too many variables are recorded at too many time points, this can lead to low participation rates, high dropout rates, and poor compliance from the volunteers. The experience is then that not all data are evaluated.
The question to be answered and the strategy for evaluation must be specified in the study protocol before the study is started.
Study population The question to be answered by the study implies that there is a target group for whom this is to be clarified. Nevertheless, the research worker is not primarily interested in the observed study population, but in whether the results can be
transferred to the target population. Accordingly, statistical test procedures must be used to generalize the results from the sample for the whole population (figure 1).
The sample can be highly representative of the study population if it is properly selected. This can be attained with defined and selective inclusion and exclusion criteria, such as sex, age, and tumor stage. Study participants may be selected randomly, for
example, by random selection through the residents’ registration office, or consecutively, for example,
all patients in a clinical department in the course of one year.
With a selective sample, a statement can only be made about a population corresponding to these selec- tion criteria. The possibility of generalizing the results may, for example, be greatly influenced by whether the patients come from a specialist practice,
a special- ized hospital department or from several different practices.
The possibility of generalization may also be influ- enced by the decision to perform the study at a single institution or site, or at several (multicenter study). The advantages of a multicenter study are that the required number of patients can be reached
within a shorter period and that the results can more readily be generalized, as they are from different treatment centers. This raises the external validity.
Type of study Before the study type is specified, the research worker must be clear about the category of research. There is a distinction in principle between research on primary data and research on secondary data.
Research on primary data means performing the actual scientific studies, recording the primary study data. This is intended to answer scientific questions and to gain new knowledge.
In contrast, research on secondary results involves the analysis of studies which have already been per- formed and published. This may include (renewed) analysis of recorded data, perhaps from a register, from population statistics, or from studies.
Another objective may be to win a comprehensive overview of the current state of research and to come to appropriate conclusions. In secondary data research, a distinction is made between narrative reviews, systematic reviews, and meta-analyses.
The underlying question to be answered also influ- ences the selection of the type of study. In primary research, experimental, clinical and epidemiological research are distinguished.
Experimental research includes applied studies, such as animal experiments, cell studies, biochemical and physiological investigations, and studies on
FIGURE 1 Connection between overall population and study population/data
186 Deutsches Ärzteblatt International⏐⏐Dtsch Arztebl Int 2009; 106(11): 184–9
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