There are various epidemiological study designs available for the clinical officials to use when investigate illnesses or injuries within a given population. However, the choice of a particular study design depends on the nature of health outcome under study, feasibility constraints and presumed relation to environmental exposure (Prentice, 2006). The basic principles of study designs that support all epidemiological research can be seen in terms of a clearly defined source population or study base (Monson, 1990).
For instance, when studying occupational risk factor for injuries and diseases, the study base should be drawn from well-defined cohort of employees. Secondly, epidemiologists seek estimate incidence as a basic measure of disease[s] occurrence (Monson, 1990). Mortality is considered as a special incidence whereby death is an event as opposed to occurrence, therefore, for fatal cases death as an event is the key indicators for an epidemiologist to use in his/her study.
In this regard, this paper examines three epidemiological study designs: cohort, studies, cross-sectional studies and case-control with respect to their characteristics, strengths and limitation. Epidemiology Study Design There are number of epidemiology study designs available for epidemiologists to use in their investigations. These study options, has various characteristics that position them less suitable or more suitable for investigating a specific disease relation (see table1) Cohort Studies
Cohort studies are one of the epidemiological study designs that examines multiple health effect of an exposure, whereby the subject are defined according to their exposure level followed for disease occurrence. Cohort study involves determination of successive incidence of health outcomes and follow-up of population. Cohort studies are usually classified on the basis of their temporary sequence giving two main classes of prospective and retrospective. Prospective cohort study involves investigation of relatively shorter phenomenon like pregnancy outcomes.
The case of pregnancy outcomes, temporary relation between the subsequent risk and exposure is short the duration of prospective cohort study may be short as a single work shift a work-week or years or decades for incidences of injuries prospective cohort study limited by challenges posed from logistical difficulties in following study subjects and updating exposure data which causes feasibility constraint (Checkoway, 2004). On the other hand retrospective cohort studies was initially designed as an alternative to prospective cohort studies, investigates diseases with latency periods and long induction.
Retrospective cohort studies are useful when studying incidences resulting from chronic diseases and mortality. However, Retrospective cohort studies are limited to mortality outcomes since mortality data is ready available as opposed to non-fatal data across may countries secondly, absent or scanty date on population past exposure is also a limitation to historical studies (Checkoway, 2004). Nevertheless, both cohort study designs have intuitive logical appeal from exposure to disease outcome in temporal sequence in cases such as random missed clinic trial.
Moreover, the shortcoming of temporality of outcome and exposure can be determined by other epidemiological study designs (Koepsell, 2003). Cross-sectional Studies This is study design that entails comparison of illness prevalence among the groups categorized according to exposure level and type or non-exposed groups. The study base in cross-sectional studies is normally determined by exposure status. Cross-sectional study is relevant and useful to epidemiologists to study persistent condition rather reversible effects of exposure.
Moreover, cross-sectional studies can be used to investigate many heath outcomes such as repetitive motion musculoskeletal disorders, pneumoconiosis and other psychological abnormalities like elevated liver enzymes, and biological damage indicators like chromosome abnormalities (Checkoway, 2004). The limitation of cross-sectional design is that it provides a limited causal inference due to the fact that in studies health outcomes and exposure are studies concurrently; therefore, such studies are frequently prone ‘‘reverse causation’ bias.
Case-control Case-control studies involve exposure comparison between reference of groups free of diseases and index case groups. In other words, this study design examines multiple exposures in relation to a disease, whereby study base is defined as cases, control and exposure histories compared. In investigation, the selection is done to only persons who meet inclusion criteria while control is created at the same time. The study cases are drawn from different sources like birth certificates, hospital registers or death certificate.
Case-control study has advantage in dimension of efficiency. For instance, rare diseases studies such as cancers, using cross-control design provide a time-and cost efficient mechanism of acquiring large number of cases that makes the health office to avoid prolonged follow-up of large groups. Additionally, reduced study size aspect facilitates efficient resources allocation to obtain data on potential confounding factors and refining exposure assessment (Checkoway, 2004).
However, control selection in community based studies is usually a complicated task, therefore, control selection usually give the epidemiologists a challenge on the criteria to use identify control group that represents source population generating the case. In conclusion, the paper has discussed three main epidemiological study designs such as Case-control, Cross-sectional Studies and Cohort Studies with respect to their characteristics, strengths and limitations.However, more need to be done to establish their effectiveness in epidemiological investigations.
References Monson, R. (1990), Occupational epidemiology: Boca Raton, CRC Press Checkoway, H. (2004), Research methods in occupational epidemiology. New York: Oxford University Press Koepsell, D. (2003), Epidemiologic methods for studying the occurrence of illness: New York, McGraw-Hill Prentice, L. (2006), “Disease prevention trials in a case-cohort