Why can you only use odds ratio in case control?

Incidence is Unknown in a Case-Control Study

In contrast, in a case-control study one can only calculate the odds ratio, i.e. an estimate of relative effect size, because one cannot calculate incidence. Consider once again the table that we used above to illustrate calculation of the odds ratio.

Is odds ratio only used in case-control studies?

Only studies using specific case-control designs should report odds ratios, whereas the case-cohort and incidence-density sampled case-control studies must report risk ratio and incidence rate ratios, respectively.

Why is odds ratio used in case-control studies instead of relative risk?

The key difference being, for odds ratio, one needs to know the conditional probabilities P(I|N),P(C|N), which are known in case control studies; whereas for relative risk, P(I),P(C) are not known from a retrospective study design. Cool!

Why not use risk ratio case-control study?

The number of persons in the control group is usually decided by the investigator. Often, the size of the population from which the case-patients came is not known. As a result, risks, rates, risk ratios or rate ratios cannot be calculated from the typical case-control study.

Why odds ratio is not used in cohort studies?

However, in cohort studies and RCTs, odds ratios are often interpreted as risk ratios. This is problematic because an odds ratio always overestimates the risk ratio, and this overestimation becomes larger with increasing incidence of the outcome.

Using Odds ratio in case control studies

Do you use odds ratio OR risk ratio for cohort study?

Odds ratios (OR) are commonly reported in the medical literature as the measure of association between exposure and outcome. However, it is relative risk that people more intuitively understand as a measure of association. Relative risk can be directly determined in a cohort study by calculating a risk ratio (RR).

Can you use odds ratio in cross-sectional study?

Odds ratio (OR) and risk ratio (RR) are two commonly used measures of association reported in research studies. In cross-sectional studies, the odds ratio is also referred to as the prevalence odds ratio (POR) when prevalent cases are included, and, instead of the RR, the prevalence ratio (PR) is calculated.

Are odds ratios the most appropriate measure of association for case-control studies?

The measure of association for a case-control study is the odds ratio (the odds of exposure to a factor among cases divided by the odds of exposure to the factor among controls). The odds ratio tells us how many times higher (or lower) the odds of exposure to a factor is among cases than among controls.

What are the limitations of a case-control study?

The main issues of concern with a case-control study are recall bias, its retrospective nature, the need for a careful collection of measured variables, and the selection of an appropriate control group.

Why is a case-control study better than a cohort study?

Thus in comparison to cohort studies, case-control studies are quick, relatively inexpensive to implement, require comparatively fewer subjects, and allow for multiple exposures or risk factors to be assessed for one outcome (Table 4).

Why is odds ratio a good estimate of relative risk?

Relative risks and odds ratios

The odds ratio is calculated in a similar way: it is simply the ratio of the odds in the two groups of interest. We know that if the odds ratio is less than one then the odds (and therefore the risk too) has decreased, and if the odds ratio is greater than one then they have increased.

What types of variables can be tested by odds ratio?

The odds ratio (OR) is a measure of association that is used to describe the relationship between two or more categorical (usually dichotomous) variables (e.g., in a contingency table) or between continuous variables and a categorical outcome variable (e.g., in logistic regression).

What are the advantages of a case-control study?

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.

What are two major limitations of case studies?

What are the strengths and weaknesses of case-control study?

What Cannot be calculated in a case-control study?

In a case-control study, you cannot measure incidence, because you start with diseased people and non-diseased people, so you cannot calculate relative risk. The case-control design is very efficient.

Can odds ratios and risk ratios both be calculated in a case-control study?

Key Concept: Remember that in a cohort study you can calculate either a risk ratio or an odds ratio, but In a case-control study: you can only calculate an odds ratio.

Can you use odds ratio in randomized controlled trials?

Odds ratio (OR) and risk ratio (RR) are the most common measures of effect size used in randomized controlled trials (RCTs) to compare the frequency of binary outcomes in the intervention and control groups [1].

Which type of study design should an odds ratio calculation be used for?

Odds ratios are most commonly used in case-control studies, however they can also be used in cross-sectional and cohort study designs as well (with some modifications and/or assumptions).

Why do we use odds ratio in cross-sectional study?

A ratio between two prevalences to analyse the association in a cross-sectional study is also a rate ratio but it is not a "relative risk" because a cross-sectional study is a point study where you see all the elements at the same time so it is not possible to talk about causality.

Can you use odds ratio in a prospective study?

Odds ratio is the key statistic for most case-control studies. In prospective studies, Attributable risk or risk difference is used to quantify risk in the exposed group that is attributable to the exposure.

What are the advantages of odds ratio?

The great value of the odds ratio is that it is simple to calculate, very easy to interpret, and provides results upon which clinical decisions can be made. Furthermore, it is sometimes helpful in clinical situations to be able to provide the patient with information on the odds of one outcome versus another.

Can you use odds ratio in retrospective cohort study?

The retrospective cohort design allows researchers to calculate odds ratios and prevalence.

Why are case-control studies better than cross sectional?

Cross sectional studies are used to determine prevalence. They are relatively quick and easy but do not permit distinction between cause and effect. Case controlled studies compare groups retrospectively. They seek to identify possible predictors of outcome and are useful for studying rare diseases or outcomes.

What are the disadvantages of a case-control study vs cohort?

However, case-control studies are less adept at showing a causal relationship than cohort studies. They are more prone to bias. One example is recall bias: cases might recall certain exposures more clearly than controls, simply due to the fact that they have thought about what could have caused their disease.