eli5 odds of getting struck by lightning

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A coworker and I were looking up the odds of getting struck by lightening today, which are about 1 in a million but getting struck twice in your life is 1 in 7 million. I can’t seem to work this out

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You might be overthinking it.

The statistic comes from how many people are reported to be struck by lighting each year. It’s about 1 in a million people (it’s actually a little less than 1 in a million). But for simplicity’s sake, that means about 1 out of every million people in the US will be struck by lightning (this statistic is US specific from the CDC).

Then for the second statistic, it’s calculated the same way. How many people are reported to have suffered a second lightning strike each year. Apparently that number is about 1 in 7 million people. So 1 out of every 7 million people in the US will suffer 2 lightning strikes in a given year.

There’s nothing else really to work out.

I’m not sure how they calculate the odds, but I do know of a local lady that has been hit twice.

It’s because some people are at higher risk than others.

People who like to hike in a thunderstorm are more likely to get struck than people who stay indoors all the time. People who live in Texas are more likely to get struck, and people who like in Alaska are less likely.

So basically if someone has been struck by lightning, it’s likely that where they live, and what they do, makes it more likely they’ll be struck again compared to someone else.

Also, please remember that your odd aren’t always 1 in a million. If you go to certain places under certain conditions your odd are much much higher.

If you are hiking on the top of a treeless ridge during and electrical storm your die role of getting his is far less than one in a million.

Same if you are out on flat water with something metal that can attract a strike.

So first of all, the probability of getting struck by lightning ***today*** is not 1 in a million – it’s much less. Humans live for about 27,000 days (conservative estimate – that’s 75 years). This means that if, each day, you had a chance of getting struck by lightning of 1 in a million, your odds of getting struck some time in your life would be about 1 in 37 (you can calculate this as 1-(1-1/1,000,000)^(27,000)). With those kinds of odds, we’d all personally know several people who had been struck by lightning.

The 1 in a million odds instead apply to a full *year*, which makes a lot more sense. With that figure, the odds of getting struck at some point in your life are about 1 in 13,000.

So how about getting struck twice? If the odds of getting struck were independent, then using the binomial distribution you can work out that the probability of getting struck two times or more in your life would be about 1 in 360 million. The real figure seems to be lower. You had it as 1 in 7 million. I’ve also seen e.g. 1 in 9 million, but in any case we’re talking about a factor 30 lower than you’d expect from the above calculation.

So what gives? Well, this indicates that the odds *aren’t* independent. In particular, to explain this outcome, some people have to be more likely to be hit by lightning than others, probably because of where they live or because of how they spend their days. There are certain risk factors for being struck that some people avoid, while others run afoul of them a lot. Like, being outdoors in wide open spaces. This means that people who have been struck by lightning once (and survived) are more likely to be struck a second time, compared to the average person, since being struck by lightning once indicates that you likely tick a box that gives you an enhanced risk.