# What are hazard ratios?

A Cox model (multivariable survival analysis) allows assessing simultaneously the effect of several factors on survival. It means that you can examine how certain factors influence the rate of a particular event happening (e.g., complication, death, ...) over time. This rate is named hazard rate.

Usually, this hazard rate is expressed as a hazard ratio which is just a mathematical transformation of the hazard rate:

Hazard Ratio = exp( Hazard Rate )

If a predictive variable is associated with a hazard ratio greater than 1, this variable increases the likelihood of the event happening. For example, if smoking is associated with an HR of 2 for the event "cardiovascular disease", it means that smoking increases the risk of this event.

# How to calculate hazard ratios?

You just need to perform a Cox model to calculate HR:

- In the Statistics menu, click on "Predictive factors (Multivariate)"
- Select the event variable you want to predict. This could be "Complication date", "Death date", ...
- Select the variables that you think are predictive and for which you want to calculate the hazard ratio
- Click on "Calculate results"
- Head to the results table

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