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Epidemiological studies

            Epidemiology can be defined as the study of the dissemination and causes of disease incidence in human populations and the relevance of this study to manage health challenges. Epidemiologic research involves various types of study designs, such as experimental studies, case-control studies, and cohort studies. Each version of epidemiological research design basically represents the diverse ways of obtaining information. The choice of one method over the other relays on specific research questions, concerns about credibility and efficiency (Aschengrau & Seage, 2008).
Experimental studies
            Experimental studies are widely known as trials, they encompass the use of formulated experiments to explore or examine the function of various agents in the causation, deterrence, or treatment of disease. They are generally implemented as intervention researches. In the experimental design of study the researcher assigns subjects of the study to two or three groups that may or might not receive any therapeutic treatment or preventive medication. The functional manipulation of the therapy by the researcher is the hallmark that differentiates experimental studies with observational studies (Aschengrau & Seage, 2008).The ability of the investigator to control the experiment and the close resemblance it has to laboratory research are the major strengths of experimental studies. On the other hand one major weakness associated with experimental studies is that studies conducted on living human beings can not achieve a high degree of control as in laboratory research. Experimental studies are guided by a statement of hypothesis. There are two types of experimental studies namely randomized controlled trials and quasi-experiments (Friis, 2010).
Cohort studies
            A cohort can be termed as populace group or a category thereof differentiated by a common trait, that is evaluated over time. There are three examples of cohort and they include age or birth, work or employment, and finally educational or school oriented cohort. Due to the rigorous scientific nature of experimental studies; they are often rendered impracticable, as a result of ethical issues, huge costs involved and difficulty in identifying participants. Cohort studies are also referred to as natural experiments, since the investigator acts as an impartial observer simply letting nature to cut its own path (Magnus, 2007). Cohort studies are generally not intentionally designed by humans. In cohort studies the major weakness is the inability of the investigator to study two or more exposures in a single study. In cohort studies the most appropriate measure of risk is relative risk. This refers to the frequency of disease or health results in the controlled study group to the frequency of the disease in the exposed group. Strengths of cohort studies include, allow direct investigation of risks and have a well defined exposure factor. While a major weakness is that they are very expensive and take long durations to conduct (Friis & Sellers, 2009).
Case-Control studies
            A case-control study refers to a situation where the subjects are described by virtue of the presence or absence of a desired outcome. The cases are those persons who posses the disease being investigated or the desired outcome, while the control does not. Use of specific criteria such as disease of interest for selection of groups limits the case-control study to single outcomes. It makes use of a retrospective approaches in data collection. One method used to define past exposure is the researcher to interview cases and controls in relation to their exposure history (Friis & Sellers, 2009).Strengths of case-control studies are that they can investigate various potential exposures. Case-control studies use the odds ratio as an evaluator of association between the incidence of exposure and the incidence of outcomes made use of in case-control studies. Frequency rates are not used; instead the hazard of a result associated with an exposure is projected by calculating the chances of exposure between the cases and controls (Magnus, 2007).
            In the cohort study of the association of pesticides to prevalence of leukemia in children, the range of units measuring exposure is too narrow. Basing of the fact that the accuracy of the methods used to measure the units is low. Faulty measurement equipment may present biases into the research design. In this study there is a significant bias in the selection of the data collection procedures. Due to the small range used in data collection, there will be no significant difference in the data collected from the exposed and control groups. The results or observations from the two groups will remain relatively unchanged (Aschengrau & Seage, 2008).
In the study carried out on lifetime sexual partners, the interviews take place after the participants have been notified of their HIV status. This brings about the Hawthorne effect, by virtue of knowing their status the participants are bound to change their responses or behavior. This will significantly underestimate or overestimate the findings in relation to a respondent’s medical status.
In this study of PCBs in the work place the interval of scheduled follow up is to long and some of the respondents may not be able to recall events as good as others leading to a recall bias. Failure to locate 15% of the respondents further undermines the credibility of the findings.
In this clinical trial there is bias in the selection of the data collection personnel. The pharmaceutical company administers the drug and interviews the subjects. The roles of the two doctors should be interchange to enhance validity.
            Regardless of the various limitations epidemiological studies play an integral role in the prevention, treatment and detection of diseases.

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