Via Bruce Schneier, there is this quite good write up on risk assessment in the government. Apparently, most government agencies actually have explicit risk metrics when allocating resources based on the chance of things causing human fatalities:
An unacceptable risk is often called de manifestis, meaning of obvious or evident concern -- a risk so high that no "reasonable person" would deem it acceptable. A widely cited de manifestis risk assessment comes from a 1980 United States Supreme Court decision regarding workers' risk from inhaling gasoline vapors. It concluded that an annual fatality risk -- the chance per year that a worker would die of inhalation -- of 1 in 40,000 is unacceptable. This is in line with standard practice in the regulatory world. Typically, risks considered unacceptable are those found likely to kill more than 1 in 10,000 or 1 in 100,000 per year.
At the other end of the spectrum are risks that are considered acceptable, and there is a fair degree of agreement about that area of risk as well. For example, after extensive research and public consultation, the United States Nuclear Regulatory Commission decided in 1986 that the fatality risk posed by accidents at nuclear power plants should not exceed 1 in 2 million per year and 1 in 500,000 per year from nuclear power plant operations. The governments of Australia, Japan, and the United Kingdom have come up with similar numbers for assessing hazards. So did a review of 132 U.S. federal government regulatory decisions dealing with public exposure to environmental carcinogens, which found that regulatory action always occurred if the individual annual fatality risk exceeded 1 in 700,000. Impressively, the study found a great deal of consistency among a wide range of federal agencies about what is considered an acceptable level of risk.
This falls down when it comes to terrorism:
As can be seen, annual terrorism fatality risks, particularly for areas outside of war zones, are less than one in one million and therefore generally lie within the range regulators deem safe or acceptable, requiring no further regulations, particularly those likely to be expensive. They are similar to the risks of using home appliances (200 deaths per year in the United States) or of commercial aviation (103 deaths per year).
Hmmm... I'm going to have to start keeping an eye out on my dishwasher. I'm pretty sure it has it in for me.