Recently I was asked a question about the difference between risk ratio and rate ratio. This article is in response to that question.
Incidence: This is the number of new cases occurring in a population during a specified time-period, and is given by the formula:
where x is a multiple of 100
Since a time element is involved, incidence is a rate. This also allows calculation of incidence density, where the denominator is the sum of all person-times. For more on this, please read https://communitymedicine4all.com/2020/07/05/measures-of-morbidity-incidence-rate-and-attack-rate/
Risk (of an outcome occurring): This is the probability of an outcome occurring. The outcome could be a disease condition, health state, etc. Generally, every member of a population has a certain probability of having a particular outcome- regardless of exposure status (exposed/ non-exposed). Let us understand this concept with the help of an example:
Consumption of tobacco products increases the risk of several diseases, notably cardiovascular diseases, and lung cancer. However, not all who consume tobacco products develop cardiovascular diseases of lung cancer. Similarly, not all who develop cardiovascular diseases or lung cancer consume tobacco products. Therefore, even in the absence of tobacco consumption (exposure factor), some people may develop cardiovascular diseases or lung cancer. This is referred to as the baseline risk of developing cardiovascular diseases or lung cancer. When examining the relationship between tobacco consumption and development of lung cancer (say), one compares the risk of developing lung cancer in those who consume tobacco with the baseline risk (those who do not consume tobacco products). From the above it must be evident that regardless of whether one consumes tobacco products, everyone has a certain risk of developing cardiovascular diseases or lung cancer. We know that the risk is higher for those who consume tobacco products, but this does not imply that those who do not consume tobacco products have zero risk. This is the underlying concept of everyone having a certain probability of having a particular outcome regardless of exposure status. People would have zero probability of having a disease only when they have lifelong immunity against that disease.