Risk Ratio: Formula, Calculation, Examples & Benefits

What is Risk Ratio?

The risk ratio involves comparing the risk of an outcome between two groups, i.e., the exposed group and the unexposed group. It is also known as the relative risk. It is very commonly used for statistical measurement, especially for clinical research in the medical field. It informs public health policies and helps policymakers prioritize health issues, allocate resources effectively, and implement strategies to reduce the risks or promote protective factors.

Geeky Takeaways:

• The exposed group consists of individuals who have been exposed to a certain factor or risk.
• The unexposed group consists of individuals who have not been exposed to the same factor or risk.
• The risk ratio allows for a direct comparison of the risk levels by comparing the risk of the outcome.

How to Calculate Risk Ratio? (Formula of Risk Ratio)

The risk ratio involves comparing the risk of an outcome between two groups: the exposed group and the unexposed group. The steps to calculate the risk ratio are as follows,

1. Calculate the Risk in the Exposed Group: The foremost step includes calculating the risk in the exposed group by counting the number of individuals in the exposed group who have experienced the outcome.

Risk in the Exposed Group = [Tex]\frac{Number~of~exposed~individuals~with~the~outcome}{Total~number~of~exposed~individuals}[/Tex]

2. Calculate the Risk in the Unexposed Group: It also comes under the first step. The researcher should calculate the risk in the unexposed group by counting the number of individuals in the unexposed group who have experienced the outcome.

Risk in the Unexposed Group = [Tex]\frac{Number~of~unexposed~individuals~with~the~outcome}{Total~number~of~unexposed~individuals}[/Tex]

3. Apply the Formula for Risk Ratio: Once the risks in both groups have been calculated, the following formula is used to calculate the risk ratio:

Formula of Risk Ratio:

Risk Ratio = [Tex]\frac{Risk~in~exposed~group}{Risk~in~unexposed~group}[/Tex]

Example of Risk Ratio

Let us take an example of a study to investigate the risk of developing type 2 diabetes among individuals with and without obesity,

• Exposed Group (Obese Individuals): There are 500 individuals with obesity.
• Unexposed Group (Non-obese Individuals): There are 1000 individuals without obesity.

During the follow-up period:

• In the exposed group (obese individuals), 100 individuals developed type 2 diabetes.
• In the unexposed group (non-obese individuals), 50 individuals developed type 2 diabetes.

1. Risk in the Exposed Group = [Tex]\frac{Number~of~exposed~individuals~with~the~outcome}{Total~number~of~exposed~individuals}[/Tex]

[Tex]Risk~in~exposed~group=\frac{100}{500}=0.20[/Tex]

2. Risk in the Unexposed Group = [Tex]\frac{Number~of~unexposed~individuals~with~the~outcome}{Total~number~of~unexposed~individuals}[/Tex]

[Tex]Risk~in~the~unexposed~group=\frac{50}{1000}=0.05[/Tex]

3. Risk Ratio = [Tex]\frac{Risk~in~exposed~group}{Risk~in~unexposed~group}[/Tex]

[Tex]Risk~Ratio=\frac{0.20}{0.05}=4[/Tex]

Since the risk ratio is 4, it indicates that individuals with obesity are 4 times more likely to develop type 2 diabetes compared to those without obesity. In other words, there is an increased risk of developing type 2 diabetes associated with obesity.

Interpretation of Risk Ratio

The interpretation of the Risk Ratio(RR) following points:

• RR = 1: When the risk ratio is equal to 1, it indicates that there is no difference in risk between the exposed group and the unexposed group. Therefore, the exposure or risk factor does not impact the likelihood of the outcome occurring.
• RR > 1: If the risk ratio is greater than 1, it suggests that the risk of an event occurring is less with the exposed group than without the exposed group. This indicates a positive relation between the exposure and the outcome. It indicates the risk factor.
• RR < 1: When the risk ratio is less than 1, it suggests a negative relation to the exposure. In other words, the exposure reduces the likelihood of the outcome occurring. It indicates the protective factor.

Benefits of Risk Ratio

1. Comparative Analysis: The risk ratio allows for a direct comparison of the risk levels by comparing the risk of the outcome between exposed and unexposed groups. This analysis is important for identifying potential risk factors or protective factors which can affect the outcome.

2. Simple Interpretation: The risk ratio is easy to interpret. If the risk ratio equals 1, it indicates no difference in risk between the exposed and unexposed groups, whereas values greater than 1 indicate an increased risk in the exposed group and values that are less than 1 indicate a decreased risk in the exposed group.

3. Communicates Risk Effectively: The risk ratio communicates the risk information to all the stakeholders and the general public. It helps in conveying the potential impact of exposures on health outcomes.

4. Useful in Studies: In the field of public health and its studies, the risk ratio helps people assess the effectiveness of treatments, interventions, or preventive measures. It helps the researchers to evaluate the risk.

5. Standardized Measure: It is a standardized measure that is widely used in epidemiological research to make it easier to compare various findings across studies and populations. This enhances the soundness and validity of research findings in the field of public health.

Limitation of Risk Ratio

1. Sensitivity to Outcome Frequency: The risk ratio is sensitive because small changes in the number of cases can lead to large fluctuations in the risk ratio. this makes the interpretation challenging, especially in small studies.

2. Inability to Establish Causality: The risk ratio quantifies the relation between the exposure and the outcome but it does not establish the possible causalities. Therefore, the researchers need to be careful while concluding the causal relationships based solely on the risk ratio.

3. Limited General Applicability: The risk ratio obtained from a particular study may not be valid to other populations. Differences in population, exposure levels, or other factors may affect the extent and direction of the risk ratio.

4. Time-to-Event Data: Risk ratios are used to compare the incidence of outcomes between groups at a single point in time. However, they may not be suitable for analyzing time-to-event data where the outcome occurs over some time. In this case, alternative measures become more suitable.

5. Inaccuracy in Rare Outcomes: The risk ratio during certain circumstances may overestimate or underestimate the true risk difference between exposed and unexposed groups which will lead to vague estimates.

Risk Ratio vs. Odds Ratio

Basis	Risk Ratio	Odds Ratio
Definition	The risk ratio is the ratio that compares the risk of an outcome between two groups: the exposed group and the unexposed group.	The odds ratio is the ratio that compares the odds of an outcome occurring in the exposed group to the odds of the outcome occurring in the unexposed group.
Applicability	It is used when the outcome is common or when the researcher is studying the effects of interventions on health outcomes.	It is used when the outcome is rare or when studying relations in cross-sectional or retrospective studies.
Outcomes	It gives the outcome of the total number of trials.	It gives the outcome about the number of outcomes which are not of our interest.
Formula	RR= Risk in the exposed group/Risk in the unexposed group	OR= Odds of outcome in exposed group/Odds of outcome in unexposed group

Risk Ratio – FAQs

What does a risk ratio of 1 mean?

The risk ratio of 1 indicates that there is no difference in risk between the exposed and unexposed groups.

When is the risk ratio used in research?

The risk ratio is used in cohort studies and trials to assess the association between exposures and health outcomes. It is particularly useful for studying the effects of risk factors on disease occurrence.

Can the risk ratio be used to establish causality?

While the risk ratio provides evidence of an association between an exposure and an outcome, it does not establish causality on its own.

How does the risk ratio differ from the odds ratio?

The risk ratio compares risks directly, while the odds ratio compares odds. The risk ratio is used when the outcome is common, whereas the odds ratio is used when the outcome is rare.