Monday, August 09, 2004


Currently on airplanes children under two can ride free by sitting on a parent's lap. The FAA is considering whether to require such children to have their own seat in a child seat similar to the ones most states require for cars. Sounds reasonable? One argument against goes as follows: If we require parents to pay for a seat for a children there is a chance they will drive instead greatly increasing their risk.

How can we evaluate risk? Decision scientists have developed a measure called micromorts (μmorts). A μmort is a one-millionth chance of death. Sounds gruesome but by counting micromorts we can analyze the right choices to keep the most people alive.

All of three lap children have died in airplane crashes where their parents survived since 1987. The average driver runs the risk of about .02 μmorts/miles. If the average car trip is say 500 miles that translates to about 10 μmorts for each child in the car. Three laptop children have died in airplane crashes where the parent has survived since 1987. This translates to the equivalent of 300,000 car trips or about 15,000/year. About 6 million children ride on laps on airplanes each year, so if more than 0.25% of them were to ride in a car instead because of the higher prices, we would about cost lives by requiring safety seats on planes. My numbers, drawn from various internet sources, don't tell the whole story but nevertheless we can and should do a full analysis before setting policy.

It would be nice to have a list of various activities and how many μmorts they use, say you feel like parachuting, you can get an idea of how dangerous it is compared to say riding a bicycle. But we don't get such lists and people have to use their own judgments and often make the wrong decisions. We can also give a cost amount to a μmort; how much is it worth to save lives?

By finding statistics online you can calculate the risks in your various activities. You need to use about 3 μmort/day on average to keep a 10% chance of accidental death in your life. Spend them wisely.


  1. Baby seat or no baby seat, I say there needs to be some economic penalty for bringing babies onto plane flights considering how unpleasant they make everyone else's trips.

  2. No, I think the statistics are correct. He's trying to determine if the decrease in risk for airline lap babies is offset by the increased risk for car babies. We assume that the special seat would make them approximately as safe as their parents are. Considering cases in the past where both the child and parent died would not be a useful statistic, because those children would be dead even if the new seats were introduced. The children who would have been saved by the new seats are precisely those who have died, but whose parents survived the crash. Using the parents is a good measure; it eliminates other factors like how far the victim's seat is from the point of impact.

    And the number of children who can be saved by the child seats gives us the decrease in risk.

  3. I think there's more than a little danger in making such subjective assessments seem so scientific. This kind of comparison falls safely under what I would call pseudo-science, used as you propose.

    Not that there isn't value in the attempt to make very subjective comparisons more quantitative, but to suppose that we can use this kind of method as the basis for an entire policy is, I think, irresponsible. At the very least, it ignores the sources of the cited statistics, along with all kinds of problems of numerical analysis.

    For example, what if I started "saving up" my micromorts for an especially dangerous activity? The first lecture of any undergraduate probability course will tell you that that kind of behavior is nonsense, but it makes perfect sense if you're trying to "spend 3 micromorts a day."

    Just some thoughts,


  4. However, the lecture on Poisson processes and hitting times should make it sensible once more...