Let’s first say what risk is! Risk can be described as a combination of the probability of occurrence of harm and the severity of that harm. Three commonly used methods could help estimate the risk: qualitative, semiquantitative and quantitative. The qualitative approach is the least preferred method, while the quantitative is highly recommendable when performing risk estimation.
 Qualitative method
There are two cases when the qualitative method is suitable to be used: a quantifiable data is unavailable, and the confidence in the available data is low. In such circumstances, the risk estimator is advised to use an NxM matrix to stratify the risks into zones (e.g. high, medium and low zones). This method relies on the usage of very good definitions for each probability and severity level that could ensure the repeatability and consistency of ratings by various analysts, at different times.
 Semiquantitative method
The semiquantitative method is generally used on existing products about which a lot of data has been collected. This method is similar to the qualitative method, but with the difference that quantifiable data is available. When using this method, the risk estimator must keep in mind that the scales for the probability of occurrence of harm are different for the different products. For instance: the probability of harm per use, the likelihood of harm per hour of use, and the probability of harm per device.
With this method, each hazard could have:
 an estimated risk based on the available quantifiable data for the probability of occurrence of harm;
 an estimated severity of the harm.
 Quantitative method
The concept of quantification of risk indicates that risk can be the product of the probability of occurrence of a hazardous situation (P1) and the likelihood of occurrence of harm (P2). This method, even though it is highly recommended, does have a negative side. Manufacturers are forced to consider the worst possible harm that could result from a hazardous situation which, thus, can distort the risk management.
An example of a quantitative method is the BMX fivelevel risk computation method, which calculates risk in five classes of harm severity: catastrophic, critical, serious, minor and negligible. This method considers the entire spectrum of harm severities and identifies the highest risk regardless of the harm severity.
Source:

Elahi, B. (2018). “Safety risk management for medical devices”. Academic Press
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