Relative risks and snipers, or, how I learned to stop worrying and live my life. The "experts" have been coming out of the woodwork to talk about the Washington DC sniper murders, currently occurring all around where I live. A lot of "advice" is being thrown around concerning what people should and shouldn't do. But can we really trust that advice and does it have any foundations? To answer this question we really need to understand the nature of risks and relative risks.
For example, I have heard some "experts" recommend that people should fill their car fuel tanks at places away from major roads. Doing so may reduce your risk - we don't really know by how much - but it will almost certainly increase other risks, such as the risk of being killed in a car accident because you spent longer on the road.
At this point most people say, "Hang on, the increased risk of a car accident is minuscule, and there is a much reduced risk associated with staying out of the sniper's fire." But this argument may not be valid. Our intuition with regards to risk assessments are notoriously poor. Essentially every study on the public understanding of risk shows that people just can't estimate levels of risk when the risks are extremely small.
I don't know which change in risk is greater - I haven't tried to work out the numbers, and I suspect that they would be very difficult to calculate with any decent precision. But it seems entirely possible that these two changes in risk could balance each other out or you could increase your risk by driving a greater distance on less familiar roads more than you decrease your sniper risk.
Another example, just to show this isn't an isolated piece of advice that has an uncertain statistical basis: Some commentators are recommending that you don't walk in straight lines as this makes it harder for a sniper to target you. Considering that a walking person is already a difficult target and the sniper seems to have chosen stationary victims in most cases, how much does this decrease your risk? Then consider what risks it increases. Walking erratically increases the chance of tripping and falling, hitting your head, and your brain hemorrhaging. It increases your chance of stumbling in front of passing traffic, perhaps to be hit by the person driving to a different fuel station, unaware that pedestrians walk so close to the road in this area...
These increased risks may seem ludicrously small but how much did you decrease the risk by walking erratically in the first place?
The point is that we can't make valid recommendations about how to change behaviour unless we try to work out these statistics. Practically any change of behaviour will result in increased risks. Do we know for sure that increase is compensated for by the risk decrease we are aiming for?
Perhaps we shouldn't be changing our behaviour at all. What are some comparable risks?
We need to make a couple of assumptions to make any meaningful comparison. Assume that the series of shooting stops, for whatever reason, in a month. Assume that an episode like this only happens in the DC region once every decade. (This is probably an overestimate of the risk but we'll stick with it for now.) Then the risk of being shot by a sniper in the DC area is about 24 in 3.5 million per decade (the 24 is the number of expected shootings in a month - again, this might be an overestimate if the shootings are now at 3 or 4 per week.) This works out to a risk level of about 7 in a million per decade.
Now for some comparisons: the chance of dying in a car accident, as the driver, is about 240 per million per decade. The chance of dying from a fall at home is about 90 per million per decade. The chance of being hit by a car, as a pedestrian, is about 15 per million per decade. The chance of being hit and killed by lightning is about 1.5 per million per decade.
So you are only slightly more likely to be shot by the sniper than hit by lightning. You are twice as likely to be hit by a car while walking down the street than being shot by the sniper. You are more than ten times as likely to trip and fall at home than being shot by the sniper. You are 35 times more likely to die in a car crash while driving than being shot by the sniper.
When you go out of the house, do you worry about dying from driving, being hit by a car, tripping over and falling to your death, or being hit by lightning?
Perhaps you want to argue with the details of the statistics but I have been aggressively overestimating the risks from the sniper and they still come out far lower than a lot of everyday activities, including ones that we have no control over.
The take home message: don't trust your instincts about risk and be careful how you change your behaviour in response. It's entirely possible that you could be increasing your risk by being too worried.
(Note: some of the data I used to estimate risks came from here. The data may not be perfect for the DC area but I would expect it to be something like this.) [David Harris: Science news]
1:11:18 AM 
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