The objective risk of an activity depends on how likely some negative event is to actually occur. The subjective risk, also known as the perceived risk, depends on what you believe about the world. If an activity’s subjective risk differs from its objective risk, then this often triggers tensions between engineers and the public. It is not sufficient to ensure that a product is safe. Engineers also need to pay attention to how new and existing technologies are perceived.
Engineers often characterize risks in quantitative terms. According to the engineering definition of risk, the risk of some unwanted event e is the product of the probability that e will occur and the value of the harm caused by e, measured in whatever unit deemed appropriate.
The precautionary principle is an influential alternative to traditional risk–benefit analysis, especially in Europe and other countries outside the United States. According to the precautionary principle, there is no need to determine what the probability of sailing into a floating container is or what the probability of a nuclear meltdown might be. All we have to establish is that some sufficiently bad outcome may occur if no precautionary measures are taken.
The risk–benefit principle and the precautionary principle draw the line between acceptable and unacceptable risks by considering the risk’s potential consequences as well as the information we have (or do not have) about the probability of those consequences. A fundamentally different approach is to argue that what matters is whether those exposed to the risk have given their informed consent to being exposed to the risk.