Study designs are used in Public Health Research Josep Vidal Alaball January 2003

 Several quantitative study designs are used in Public Health Research and several classifications of these studies had been proposed. Probably the more widely used is the one described by Beaglehole (Beaglehole et al., 1993, pp 29-43):

 1. Observational studies   Descriptive studies   Analytical studies a. Ecological b. Cross-sectional c. Case-control d. Cohort  2. Experimental studies a. Randomised control trials b. Field trails c. Community trails

 Other authors (Burt Gerstman, 1998, pp ) do not distinguish between analytical and descriptive studies as most of the studies used in Epidemiology try to develop and generate hypothesis at the same time, the author says that “both descriptive and analytic epidemiologic research represent a continuum of design and intent”. To simplify, the classification of non-experimental and experimental studies has been also used.  In this assessment I am going to describe 6 of these study designs, comparing 3 of them.

ECOLOGICAL STUDIES  These studies are different from the rest, as the units studied are not individuals; in these studies the unit of analysis are populations.  The main advantage of these studies is that they are easy and cheap to conduct but the big disadvantage is that they are difficult to interpret and inaccurate or false conclusions can be obtained (Beaglehole et al., 1993, pp 32-3).

 They are suitable when we want to investigate a rare disease as they produce the initial data from which we can start the epidemiological research. They look at exposure.  An example of this study is the positive relationship between air pollution and mortality in people over 70 years found in Valencia (Spain), this relation is similar to the one found in other cities (Ballester et al., 1996).

CROSS-SECTIONAL STUDIES  These studies compare a group of people with the disease (cases) with a group or groups of people without the disease (controls). We use the odds ratio (OD) to calculate the association. It is easy to identify cases but the challenge is to identify suitable controls as they should have the same characteristics know or though to be associated with the disease.  They have been called retrospective studies as the data collection is usually retrospective but in some studies the data collection can be concurrent to the study.

 They are good studies to investigate rare diseases or to study multiple exposures and they are relatively cheap and less time consuming but they are difficult to design (selection of controls) and can not determine the frequency of disease. They also rely on recall with the dangers and errors that this implies (Beaglehole et al., 1993, pp 34-6).  An example of a case-control study is the one showing a relation between control of blood pressure with treatment and risk of stroke (Du et al., 1997), they concluded control of blood pressure below 150/90 prevents risk of stroke.

COHORT STUDIES  In these studies, also called Incidence Studies, a population (cohort) free of disease is identified and followed though time. We study how exposure to risks factors produces or not produces new occurrence of diseases (Burt Gerstman, 1998, pp 146-8).  They have been called prospective studies but they can be historical.

 They are good when studying rare exposures and testing multiple effects of cause but poor when studying rare diseases. They are the best testing incidence and measuring time relationship. The main disadvantages of these studies are that they are lengthy and therefore expensive (Beaglehole et al., 1993, pp 36-9).  As an example I will mention a retrospective cohort study by Raines and Wight studying mortality in nursing home admissions from hospitals compared with mortality of admissions from the community, which showed no difference (Raines and Wight, 2002).

RANDOMIZED CONTROL TRIALS (RCT)  These experimental studies are considered the most scientifically rigorous. In clinical trials, the subjects of the population under study are randomly allocated to receive or not to receive the intervention, in community trials, they are the groups, which are randomly allocated. The 2 groups are called “study” and “control group”.

 The have some limitations as they are expensive and often unethical to conduct. They may be difficult to generalize to the larger population (Burt Gerstman, 1998, pp 152-5).  Examples of RCTs can be found in all scientific publications. As an example of a community randomised trial I would like to mention the trials studying the effect of zinc supplementation during diarrhoea in Bangladeshi children, which showed reduced morbidity and mortality rates among children receiving the intervention (Baqui et al., 2002).

FIELD TRAILS  In this studies people participating are disease free but though to be at risk. The “purpose is to prevent the occurrence of diseases that may occur with relatively low frequency” (Beaglehole et al., 1993, pp 41-2). They have been used as a way to assess interventions to reduce exposure.  These are very big studies and very costly and often a previous large screening is needed to identify people eligible for the study.

 An example of this type of study is the one done in Tonga trying to improve periodontal health of adults, which showed that dental health education significantly improved periodontal health (Cutress et al., 1991).

REFERENCES  Arason, V., Kristinsson, K., Sigurdsson, J., Stefansdottir, G., Molstad, S. and Gudmundsson, S. (1996) Do antimicrobials increase the carriage rate of penicillin resistant pneumococci in children? Cross sectional prevalence study. BMJ, 313,  Ballester, F., Corella, D., Perez-Hoyos, S. and Hervas, A. (1996) Air pollution and mortality in Valencia, Spain: a study using the APHEA methodology. Journal of Epidemiology & Community Health, 50,  Baqui, AH., Black, RE., El Arifeen, S., Yunus, M., Chakraborty, SA., Vaughan, JP. (2002) Effect of zinc supplementation started during diarrhoea on morbidity and mortality in Bangladeshi children: community randomised trial. BMJ, 325,  Beaglehole, R., Bonita, R. and Kjellstrom, T. (1993) Basic Epidemiology, World Health Organization, Geneva.  Burt Gerstman, B. (1998) Epidemiology Kept Simple. An Introduction to Modern Epidemiology, Wiley-Liss, New York.  Cutress, T. W., Powell, R. N., Kilisimasi, S., Tomiki, S. and Holborow, D. (1991) A 3-year community-based periodontal disease prevention programme for adults in a developing nation. International Dental Journal., 41,

REFERENCES  Du, X., Cruickshank, K., McNamee, R., Saraee, M., Sourbutts, J., Summers, A., Roberts, N., Walton, E., Holmes, S. (1997) Case-control study of stroke and the quality of hypertension control in northwest England. BMJ, 314,  Raines, JE and Wight, J. (2002) The mortality experience of people admitted to nursing homes. Journal of Public Health Medicine, 24,

 Dr. Josep Vidal Alaball  Cardiff. January 2003 