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**.## When should you not use odds ratio?

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**. Thus there is a danger that if odds ratios are interpreted as though they were relative risks then they may mislead.## 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%.

## What is odds of disease ratio?

A relative risk or odds ratio greater than one indicates an exposure to be harmful, while a value less than one indicates a protective effect.

**RR = 1.2 means exposed people are 20% more likely to be diseased**, RR = 1.4 means 40% more likely. OR = 1.2 means that the odds of disease is 20% higher in exposed people.## How do you convert odds ratio to prevalence ratio?

For example, if 80 out of 100 exposed subjects have a particular disease and 50 out of 100 non-exposed subjects have the disease, then the odds ratio (OR) is

**(80/20)/(50/50) = 4**. However, the prevalence ratio (PR) is (80/100)/(50/100) = 1.6.## How do you interpret prevalence ratio less than 1?

A risk ratio or rate ratio of less than 1.0

**indicates a negative association between the exposure and outcome in the exposed group compared to the unexposed group**. In this case, the exposure provides a protective effect.## What is the difference between prevalence and risk?

In this analogy, prevalence is the proportion of the tub (the total population) filled with any water (prevalent cases and incident cases). Risk is the proportion of the tub filled with new, flowing water (incident cases). Rate is a measure of how quickly the water flows into the tub.

## Frequently Asked Questions

#### What does an odds ratio of 2.5 mean?

For example, OR = 2.50 could be interpreted as

**the first group having “150% greater odds than” or “2.5 times the odds of” the second group**.#### How do you know when to use odds ratio?

When is it used? Odds ratios are used to compare the relative odds of the occurrence of the outcome of interest (e.g. disease or disorder), given exposure to the variable of interest (e.g. health characteristic, aspect of medical history).

#### Why use prevalence ratio in cross-sectional study?

Since cross-sectional studies are particularly useful for investigating chronic diseases (e.g. prevalence of AIDS) where the onset of disease is difficult to determine, or for studying long lasting risk factors (such as smoking, hypertension, and high fat diets), the prevalence odds ratio will generally be the ...

## FAQ

- How does the odds ratio compare to the estimate of the relative risk Why are they so different?
- The relative risk (also known as risk ratio [RR]) is the ratio of risk of an event in one group (e.g., exposed group) versus the risk of the event in the other group (e.g., nonexposed group).
**The odds ratio (OR) is the ratio of odds of an event in one group versus the odds of the event in the other group**. - What is the difference between prevalence ratio and prevalence odds ratio?
- 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**. - When can you use odds ratio?
- 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 "prevalence odds ratio"

How do you know when to use relative risk vs odds ratio? | The relative risk (also known as risk ratio [RR]) is the ratio of risk of an event in one group (e.g., exposed group) versus the risk of the event in the other group (e.g., nonexposed group). The odds ratio (OR) is the ratio of odds of an event in one group versus the odds of the event in the other group. |

Is prevalence a rate OR ratio? | Prevalence is determined by the size of the population and the number of outcome-positive cases. Prevalence is sometimes referred as the prevalence rate, but prevalence is actually a proportion. It can never be less than zero or greater than one. |

How do you interpret odds ratio? | Important points about Odds ratio:
OR >1 indicates increased occurrence of an event. OR <1 indicates decreased occurrence of an event (protective exposure) Look at CI and P-value for statistical significance of value (Learn more about p values and confidence intervals here) In rare outcomes OR = RR (RR = Relative Risk) |

- Is prevalence an effect measure?
- In prevalence case–control studies
**the prevalence odds ratio is the standard effect measure**, just as in an incidence case–control study the (incidence) odds ratio is the standard effect measure (Morgenstern and Thomas 1993; Pearce 1998).

- In prevalence case–control studies
- Should I use risk ratio or odds ratio?
- “Risk” refers to the probability of occurrence of an event or outcome. Statistically, risk = chance of the outcome of interest/all possible outcomes.
**The term “odds” is often used instead of risk**. “Odds” refers to the probability of occurrence of an event/probability of the event not occurring.

- “Risk” refers to the probability of occurrence of an event or outcome. Statistically, risk = chance of the outcome of interest/all possible outcomes.