The aim behind randomisation or random allocation of treatment is to produce comparable treatment and control groups. This is the reason why the randomised controlled trial is such a powerful study design - other epidemiological studies depend on the measurement and statistical analysis of known confounders. Statistical techniques may not always control confounding adequately, especially if the confounding factor is not measured very well.
Placebos or similar looking treatments aim to conceal the identity of different treatments both from the patient and from the person who assesses outcome. Such masking is more difficult in surgical trials but is not a good reason to avoid randomised controlled trials of surgical interventions.
The most important design feature of a trial is to ensure that the allocation of the treatment groups is concealed. This is always possible. Masking the assessment of outcome, which is sometimes difficult in surgical trials, is not essential.
There are several good examples of trials of cataract surgery in developing countries that are providing useful information on the risks and benefits of the different types of intraocular lenses. It is very important that all trials conducted are reported properly in the medical literature.
Several international collaborative efforts are underway to ensure that all data collected during trials is made easily available to people making decisions about health care. The CONSORT statement consists of a checklist and flow diagram, intended to assist authors, editors, and reviewers by ensuring that information relevant to the trial is included in any study report. The reason for conducting trials is to improve patient care.
In order for this to happen, patients and doctors need to know the results of these trials. The medical literature is huge and it is difficult to keep up-to-date with all relevant research. The Cochrane Collaboration aims to summa-rise systematically randomised controlled trials of the effect of health care interventions, and to make these reviews accessible.
This means that everyone making decisions about health care, whether patients, doctors or healthcare providers and purchasers, can have available the best possible evidence to assist in their choice.
Bias may be defined as any systematic error in a study that results in an incorrect result. Both bias and confounding can be thought of as alternative explanations for the observed effect. National Center for Biotechnology Information , U. Journal List Community Eye Health v. Official websites use.
Share sensitive information only on official, secure websites. Epidemiology is the branch of medical science that investigates all the factors that determine the presence or absence of diseases and disorders. Epidemiological research helps us to understand how many people have a disease or disorder, if those numbers are changing, and how the disorder affects our society and our economy.
The epidemiology of human communication is a rewarding and challenging field. Much of the data that epidemiologists collect comes from self-report—from answers provided by people participating in a study. Similarly, in a trial to prevent onset of diabetes among high-risk individuals, investigators randomly assigned enrollees to one of three groups — placebo, an anti-diabetes drug, or lifestyle intervention. At the end of the follow-up period, investigators found the lowest incidence of diabetes in the lifestyle intervention group, the next lowest in the anti-diabetic drug group, and the highest in the placebo group.
In an observational study, the epidemiologist simply observes the exposure and disease status of each study participant. The two most common types of observational studies are cohort studies and case-control studies; a third type is cross-sectional studies. Cohort study. A cohort study is similar in concept to the experimental study. In a cohort study the epidemiologist records whether each study participant is exposed or not, and then tracks the participants to see if they develop the disease of interest.
After a period of time, the investigator compares the disease rate in the exposed group with the disease rate in the unexposed group. The unexposed group serves as the comparison group, providing an estimate of the baseline or expected amount of disease occurrence in the community.
If the disease rate is substantively different in the exposed group compared to the unexposed group, the exposure is said to be associated with illness. The length of follow-up varies considerably.
In an attempt to respond quickly to a public health concern such as an outbreak, public health departments tend to conduct relatively brief studies. On the other hand, research and academic organizations are more likely to conduct studies of cancer, cardiovascular disease, and other chronic diseases which may last for years and even decades.
The Framingham study is a well-known cohort study that has followed over 5, residents of Framingham, Massachusetts, since the early s to establish the rates and risk factors for heart disease. An alternative type of cohort study is a retrospective cohort study. In this type of study both the exposure and the outcomes have already occurred.
Just as in a prospective cohort study, the investigator calculates and compares rates of disease in the exposed and unexposed groups.
Retrospective cohort studies are commonly used in investigations of disease in groups of easily identified people such as workers at a particular factory or attendees at a wedding. For example, a retrospective cohort study was used to determine the source of infection of cyclosporiasis, a parasitic disease that caused an outbreak among members of a residential facility in Pennsylvania in Case-control study.
In a case-control study, investigators start by enrolling a group of people with disease at CDC such persons are called case-patients rather than cases, because case refers to occurrence of disease, not a person. As a comparison group, the investigator then enrolls a group of people without disease controls.
Investigators then compare previous exposures between the two groups. The control group provides an estimate of the baseline or expected amount of exposure in that population. If the amount of exposure among the case group is substantially higher than the amount you would expect based on the control group, then illness is said to be associated with that exposure.
The study of hepatitis A traced to green onions, described above, is an example of a case-control study. The key in a case-control study is to identify an appropriate control group, comparable to the case group in most respects, in order to provide a reasonable estimate of the baseline or expected exposure. Cross-sectional study.
0コメント