## Thursday, May 3, 2018

### The Odds of Disaster

While wandering around the internet, I ran across an articlethat dealt with the probability of a major civil disorder occurring in the USA. The author is a hydrologist, someone who studies and predicts flooding, so he used the common “100 year flood” as an example to show how the math works. I'm not going to rehash his entire article, but I do want to delve into the math a bit which means I'm going to use his example to get started.

Many of us who have bought houses have had to look into the nearby flood plains. I live about 10 miles from the Missouri River, which means that finding a bank willing to give a mortgage is going to depend on the house being on the “dry” side of the flood plain. The 100 year line is about a mile from my house, but the 500 year line cuts through my backyard. We had a scare in 2011 when the Missouri River got out of control, but we got through it.

Flood insurance exists, but it's expensive and only helps with the rebuilding and not the surviving of any flood. The Army Corps of Engineers (not your friends, but that's a topic for another time) decides where the high water mark will be for various flood events, but the bankers look at the historic data and determine the probability of water reaching a specific location over the course of a mortgage. This may sound like it has very little to do with prepping, but bear with me and I'll make the connection:
• If a house sits inside the 100 year flood plain, that means that there is a 1% chance that flood waters will get to it in any given year.
• If you're living there for 30 years the percent chance of a flood isn't cumulative (30%); the risk is the same every year, but the probability of a flood happening at some time during those 30 years is going to be >1%.
• The method for finding the probability (P) of an event over time is known as a Bernoulli Process
• Using the 1% chance for a flood each year, the equation looks like this:
• P(100 year flood)= P(F)= 1%= 0.01,
• so the probability of not having a flood is P(F')= 1- 0.01=0.99=99%
• If we look at the probability over time or occurrences, we have to multiply the chances of staying dry.
• P(no flood for 2 years)= P(F')x P(F')= (P(F'))^2=0.99x0.99= 0.9801
• and P(no flood for 30 years)= (P(F'))^30= 0.7394
• or about a 74% chance that there won't be a flood.
• That leaves (1-0.74= 0.26) a 26% chance that there will be a flood some time in that 30 year period. Not very reassuring to a banker.
The author of the article used the same method to find the probability of basically another civil war in the USA. Go read the article for his conclusions; I just I want to point out that we can use this fairly simple method to help prioritize our preps. Common sense tells me that I'm better off preparing to deal with a tornado (common around here) rather than spending my time and money getting ready to deal with a tidal wave (if one reaches me in Iowa we're all dead anyway), but when it come down to deciding between urban riots and major earthquakes (similar types of disruptions but different responses) I need to look at the probability of each and prioritize my preps. I'm going to use myself as an example; as always, YMMV and you're going to have to do the math for your own individual location.

Let's look at riots first. Believe it or not, there have been a few close enough that they'd have an effect on me where I live. Looking back at the historical data for the last 100 years (not much that can be called urban around here before that), I found 3 race riots in the late 1960s, another one back in 1919, and three civil disturbances that were almost riots that would have impacted my daily life. I'm not having any luck finding hard data on the deployment of the National Guard during the Depression of the 1930s, but I know of at least on judge that was on his way to be lynched (he was the one signing the foreclosure papers on farms) that was rescued by armed forces.

Let's just stick to the 7 instances in 100 years. That means that there is a 7% chance of a major incident in my area for any given year. Since I expect to live another 25 years (I hope), that means that the probability of me having to deal with something like a riot (P(R)) is:
• P(R)=7%=0.07
• P(R')= 1- P(R) =93%=0.93
• P(R' 25 years)= 0.93^25=0.1630=16.3%
• so, P(R)= 1-0.1630= 0.837= 83.7%
That's a pretty good chance that I'm going to see crowds burning cars and trashing buildings close to where I live before I die.

Now let's look at earthquakes. The local geological survey has recorded 12 earthquakes in the last 150 years, the strongest being a Mercalli VI or a 5-6 on the Richter scale. (Yeah, Iowa is seismically stable.) That works out to 0.08 earthquakes per year (8% chance). Using the same 25 years that I expect to still be around, the probability of dealing with an earthquake (P(E)) is:
• P(E)=0.08
• P(no earthquake)= P(E')= 1-0.08= 0.92
• P(E' 25 years)= 0.92^25 =0.124 =12.4%
• so P(E)= 1-0.124 = 0.876 =87.6%
As you can see, the odds are about the same for these two events occurring in the next 25 years, with the earthquake being a little bit more likely. This means that I'd probably be better off leaning towards preparing to respond to a minor earthquake in the area (mostly rendering aid) rather than getting ready to deal with urban riots (avoiding travel and possibly not working for quite a while).

Statistics and probability are handy tools, but remember that there are also “black swan” events, which are events that nobody foresees. It's a reference to the fact that all swans are white until you see a black one, and that changes your world-view at least a little.

Do some research on your area. If you live along one of the coasts, figure out how likely it is that a hurricane will impact your area in the time that you expect to live there. Check the weather history and see how likely a severe winter or summer will be so you can plan your heating or cooling options. Use all of the tools you can find to make your prepping easier and more tailored to your needs.