Position Sizing with the Kelly Criterion

The Kelly Criterion is a mathematical formula to determine the optimal dollar amount to bet in a given wager or investment. Say you’re offered a bet where you are a 60% favorite to win and it pays 2:1 in your favor—Kelly suggests betting 40% of your net worth. If you were offered this exact bet a million times, betting 40% of your net worth each attempt (adjusting as you go) would net you the most amount of money in the end. Thus, 40% is the optimal bet size given those odds and payouts.

I’m betting no one (myself included) would actually bet 40% of their net worth in the above scenario—even if they knew the odds and payouts were legit. Most of you probably read that previous paragraph and thought 40% was insanity. I think one of the more interesting takeaways from the Kelly Criterion is how few people live their life in a way that optimizes their net worth over the long-term (again, myself included). And most of us aren’t a little off the mark, we’re not even in the ballpark of what’s optimal. If the above offer was made to all Americans, the average bet size would probably be less than 1% of each person’s net worth (with many doing a nominal amount like $10 per bet).
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How much do we really know about our investments?

I’m a big space nerd. To the point that for my birthday two years ago my girlfriend flew us down to Houston to tour the Johnson Space Center (spoiler alert: it was awesome). Some of my favorite books the past few years have either been about the Apollo missions or how we’re going to get to Mars. So you can imagine how excited I was last year when we were out getting drinks with friends and one of the friends-of-a-friend was a woman who works at NASA. I basically bombarded her with questions the entire night.

One of the main things I was asking her about was what obstacles they still need to overcome to be able to get to Mars. She was giving me some pushback on what I thought I knew about space radiation and I remember starting the next sentence “I know [random blurb about space radiation].”As soon as I was done blabbering I thought to myself “why the hell did I just start that sentence with ‘I know’? I’m talking to a freaking NASA scientist about space travel and all I’ve done is read some books and online articles about the topic. That hardly counts as knowing.”
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