The risk value should be based on data collected over many years or test drops. In essence, the risk value for an event should be based on a large set of measurements and observations. Unfortunately, this is not always the case. Perceptions Risk assessment is often based on subjective perceptions of risk. Events that are voluntary and within your control are given higher risk values than events that are mandatory and beyond your control.
Risks are higher for events that have a dreaded result or catastrophic consequence.
Risks in sports and occupations that have fatalities are higher than sports and occupations that don't generally have fatalities. Maturity of a technology affects risk perceptions, too. Nuclear power plants have higher risks than coal-powered plants because so much more is known about burning coal than nuclear energy. System complexity will increase risks. Complex systems are harder to understand.