Case control study statistics definition

Eisenhardt 's methodological work. Yin 's guidelines and making positivist assumptions.

Case control study statistics definition

Introduction A case-control study is designed to help determine if an exposure is associated with an outcome i. In theory, the case-control study can be described simply.

Case control study statistics definition

First, identify the cases a group known to have the outcome and the controls a group known to be free of the outcome. Then, look back in time to learn which subjects in each group had the exposure scomparing the frequency of the exposure in the case group to the control group. By definition, a case-control study is always retrospective because it starts with an outcome then traces back to investigate exposures.

When the subjects are enrolled in their respective groups, the outcome of each subject is already known by the investigator.

Study Design - Case Control

Advantages of Case-Control Studies Case-control studies have specific advantages compared to other study designs. They are comparatively quick, inexpensive, and easy.

They are particularly appropriate for 1 investigating outbreaks, and 2 studying rare diseases or outcomes. An example of 1 would be a study of endophthalmitis following ocular surgery. When an outbreak is in progress, answers must be obtained quickly. An example of 2 would be a study of risk factors for uveal melanoma, or corneal ulcers.

Since case-control studies start with people known to have the outcome rather than starting with a population free of disease and waiting to see who develops it it is possible to enroll a sufficient number of patients with a rare disease.

The practical value of producing rapid results or investigating rare outcomes may outweigh the limitations of case-control studies. Because of their efficiency, they may also be ideal for preliminary investigation of a suspected risk factor for a common condition; conclusions may be used to justify a more costly and time-consuming longitudinal study later.

Cases Consider a situation in which a large number of cases of post-operative endophthalmitis have occurred in a few weeks. The case group would consist of all those patients at the hospital who developed post-operative endophthalmitis during a pre-defined period.

The definition of a case needs to be very specific: Within what period of time after operation will the development of endophthalmitis qualify as a case — one day, one week, or one month? Will endophthalmitis have to be proven microbiologically, or will a clinical diagnosis be acceptable?

Clinical criteria must be identified in great detail. If microbiologic facilities are available, how will patients who have negative cultures be classified? How will sterile inflammation be differentiated from endophthalmitis? Controls Controls should be chosen who are similar in many ways to the cases.

The selected control group must be at similar risk of developing the outcome; it would not be appropriate to compare a group of controls who had traumatic corneal lacerations with cases who underwent elective intraocular surgery.

Bulletin of the World Health Organization

In our example, controls could be defined as patients who underwent elective intraocular surgery during the same period of time. Matching Cases and Controls Although controls must be like the cases in many ways, it is possible to over-match. Over-matching can make it difficult to find enough controls.

Also, once a matching variable has been selected, it is not possible to analyse it as a risk factor. Matching for type of intraocular surgery e.

An important technique for adding power to a study is to enroll more than one control for every case. For statistical reasons, however, there is little gained by including more than two controls per case. Collecting Data After clearly defining cases and controls, decide on data to be collected; the same data must be collected in the same way from both groups.

Care must be taken to be objective in the search for past risk factors, especially since the outcome is already known, or the study may suffer from researcher bias. Although it may not always be possible, it is important to try to mask the outcome from the person who is collecting risk factor information or interviewing patients.

Sometimes it will be necessary to interview patients about potential factors such as history of smoking, diet, use of traditional eye medicines, etc. It may be difficult for some people to recall all these details accurately.

Furthermore, patients who have the outcome cases are likely to scrutinize the past, remembering details of negative exposures more clearly than controls.

This is known as recall bias. Anything the researcher can do to minimize this type of bias will strengthen the study. Analysis; Odds Ratios and Confidence Intervals In the analysis stage, calculate the frequency of each of the measured variables in each of the two groups.

As a measure of the strength of the association between an exposure and the outcome, case-control studies yield the odds ratio. An odds ratio is the ratio of the odds of an exposure in the case group to the odds of an exposure in the control group.

It is important to calculate a confidence interval for each odds ratio.Find basic statistics about arthritis, such as prevalence, disabilities and limitations, quality of life, and costs. Note: There are different data sources for some of the arthritis related statistics; therefore, case definitions and terminology will also vary.

Categorizing Statistics. The study of statistics can be categorized into two main branches. These branches are descriptive statistics and inferential statistics. C-reactive protein (CRP) a protein that is produced in the liver in response to is a biomarker of inflammation that is strongly associated with the risk of cardiovascular events, such as myocardial infarction and stroke.

Calcification the process of deposition of calcium salts. In the formation of bone this is a normal condition. Cohort Study Definition A study design where one or more samples (called cohorts) are followed prospectively and subsequent status evaluations with respect to a disease or outcome are conducted to determine which initial participants exposure characteristics (risk factors) are associated with it.

prospective study an epidemiologic study in which the groups of individuals (cohorts) are selected on the bases of factors that are to be examined for possible effects on some outcome.

For example, the effect of exposure to a specific risk factor on the eventual development of a . Case-control study designs are used to estimate the relative risk for a disease from a specific risk factor.

The estimate is the odds ratio, which is a good estimate of the relative risk especially when the disease is rare.

Case-control studies are useful when epidemiologists to investigate an.

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