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# How to obtain odds ratios from rcts if not given

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## How to obtain odds ratios from rcts if not given

Hey there, fellow health enthusiasts! Are you intrigued by the fascinating world of clinical trials and their outcomes? Well, today we're going to dive into the topic of obtaining odds ratios from Randomized Controlled Trials (RCTs). Don't worry, we'll keep it light, fun, and informative! So, what's an odds ratio anyway? It's a statistical measure that helps us understand the likelihood of an event occurring in one group compared to another. In the context of RCTs, odds ratios provide valuable insights into treatment effectiveness and can guide decision-making in the medical field. Now, let's get to the exciting part - how to obtain odds ratios from RCTs if they're not given. Here are a few recommendations to help you on your data-hunting adventure: 1. Review the RCT's methodology: Before diving into the results, it's crucial to understand the study's design and methods. Pay close attention to the sample size, randomization process, and any potential biases that may affect the outcomes. 2. Identify the outcome of interest: Odds ratios are calculated by comparing the odds of an event occurring in one group to another. Determine which outcome you're interested in (e.g., treatment success, side effects, remission rates) and find the

## What is the theory of odds ratio?

The odds ratio (OR) is a measure of how strongly an event is associated with exposure. The odds ratio is a ratio of two sets of odds: the odds of the event occurring in an exposed group versus the odds of the event occurring in a non-exposed group. Odds ratios commonly are used to report case-control studies.

## What study is odds ratio derived from?

Case-control studies 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).

## What is the formula for odds ratio?

This calculator uses the following formulae to calculate the odds ratio (or) and its confidence interval (ci). or = a*d / b*c, where: a is the number of times both A and B are present, b is the number of times A is present, but B is absent, c is the number of times A is absent, but B is present, and.

## Are odds ratios biased?

However, when there are few study participants at the outcome and covariate levels, the models lead to bias of the odds ratio (OR) estimated using the maximum likelihood (ML) method. This bias is known as sparse data bias, and the estimated OR can yield impossibly large values because of data sparsity.

## Is odds ratio a good estimate of relative risk?

Odds ratios are hard to comprehend directly and are usually interpreted as being equivalent to the relative risk. Unfortunately, there is a recognised problem that odds ratios do not approximate well to the relative risk when the initial risk (that is, the prevalence of the outcome of interest) is high.

## Frequently Asked Questions

#### What does the odds ratio indicate?

What is an odds ratio? An odds ratio (OR) is a measure of association between an exposure and an outcome. The OR represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure.

#### When an odds ratio is used to estimate the relative risk quizlet?

When can OR be used to estimate RR? The odds ratio always approximates the relative risk if the disease is frequent. In a cohort study of obesity and myocardial infarction, the odds ratio was calculated to be 4.5 while the relative risk was 2.5.

#### What does 0.2 odds ratio mean?

An odds of 0.2 however seems less intuitive: 0.2 people will experience the event for every one that does not. This translates to one event for every five non-events (a risk of one in six or 17%).

#### What does odds ratio of 1.5 mean?

As an example, if the odds ratio is 1.5, the odds of disease after being exposed are 1.5 times greater than the odds of disease if you were not exposed another way to think of it is that there is a 50% increase in the odds of disease if you are exposed.

#### Under what conditions would odds ratio be a good approximation for relative risk?

The probability of the event of interest is small (< 0.1). This condition guarantees that the odds ratio will make a good approximation to the relative risk. In this example, the event of interest is a response to the mailing.

## FAQ

When can odds ratio estimate relative risk?
When the risks (or odds) in the two groups being compared are both small (say less than 20%) then the odds will approximate to the risks and the odds ratio will approximate to the relative risk.
What is a relative risk ratio of 3?
For example, a risk ratio of 3 for a treatment implies that events with treatment are three times more likely than events without treatment. Alternatively we can say that treatment increases the risk of events by 100 × (RR – 1)% = 200%.
What is a good odds ratio?
An odds ratio greater than 1 implies there are greater odds of the event happening in the exposed versus the non-exposed group. An odds ratio of less than 1 implies the odds of the event happening in the exposed group are less than in the non-exposed group.
Why are the relative risk and odds ratio approximately equal?
If the outcome is rare in both exposed and unexposed persons, the odds ratio ([A/B]/[C/D]) will approximate the risk ratio ([A/(A + B)]/[C/(C + D)]). However, when the study outcome is common and the risk ratio is not close to 1, the odds ratio will be further from 1 compared with the risk ratio.
Is relative risk the same as rate ratio?
Risk ratio: ratio of the risk of an event in one group (exposure or intervention) to that in another group (control). So it depends on your definitions of rate and risk. The term "relative risk" is sometimes used as a synonym for risk ratio, and rate ratio is one of the relative risk measures too.

## How to obtain odds ratios from rcts if not given

 How do you calculate relative risk from odds ratio? Thus the odds ratio is (a/b) / (c/d) which simplifies to ad/bc. This is compared to the relative risk which is (a / (a+b)) / (c / (c+d)). If the disease condition (event) is rare, then the odds ratio and relative risk may be comparable, but the odds ratio will overestimate the risk if the disease is more common. When interpreting a relative risk or odds ratio if the calculation is equal to OR less than 1.0 then which of the following is true? The odds ratio is interpreted in the same manner as the risk ratio or rate ratio with an OR of 1.0 indicating no association, an OR greater than 1.0 indicating a positive association, and an OR less than 1.0 indicating a negative, or protective association. What is relative risk equal to? The relative risk (RR) or risk ratio is the ratio of the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group. What happens when an odds ratio is used to estimate the relative risk? The odds ratio will always overstate the case when interpreted as a relative risk, and the degree of overstatement will increase as both the initial risk increases and the size of any treatment effect increases. Would you say that your odds ratio is an accurate approximation of the risk ratio? As a result, risks, rates, risk ratios or rate ratios cannot be calculated from the typical case-control study. However, you can calculate an odds ratio and interpret it as an approximation of the risk ratio, particularly when the disease is uncommon in the population.
• Why would you use an odds ratio in a case-control study and not a relative risk ratio?
• Firstly, RRs cannot be applied in all cases. Because risk in a sample is an estimate of risk in a population, the sample must be reasonably representative of the population. As such, case-control studies, by simple virtue of the fact that ratios of outcomes are controlled, cannot have a risk ratio reported.
• What are the limitations of odds ratio?
• The authors illustrate that a single measure of association such as an odds ratio does not meaningfully describe a marker's ability to classify subjects. Appropriate statistical methods for assessing and reporting the classification power of a marker are described.
• When can odds ratio approximates relative risk?
• Odds ratios often are mistaken for relative risk ratios. 2,3 Although for rare outcomes odds ratios approximate relative risk ratios, when the outcomes are not rare, odds ratios always overestimate relative risk ratios, a problem that becomes more acute as the baseline prevalence of the outcome exceeds 10%.
• Under which conditions are the values for the relative risk and odds ratio the most similar?
• If there's absolutely no difference between the groups in the probability of an outcome, then both the OR and the RR are 1.0. That's the only situation in which they can be exactly equal.
• Why do we use odds ratio over relative risk?
• When the outcome is not rare in the population, if the odds ratio is used to estimate the relative risk it will overstate the effect of the treatment on the outcome measure. The odds ratio will be greater than the relative risk if the relative risk is greater than one and less than the relative risk otherwise.

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February 8, 2024
February 8, 2024
February 8, 2024