Coin toss is not a 50/50 chance. What the?

The toss of a coin. It’s an act synonymous with the concept of a 50/50 chance. The phrase, ‘it’s coin toss ‘ is even used to describe a 50-50 chance.

A coin toss is used to pick who bats first in cricket. Which side of a field a football team starts on. It’s the universal argument settler of last resort. Trusted by all.  Because as everyone knows, there is a 50/50 chance whether the coin lands heads or tails.

Ready for your world to be turned upside down? For your mind to be completely blown?

Turns out there is actually a bias in a coin flip. It’s not a 50/50 event. No, we’re not talking about the game Crazy Coin Flip which has a rather large bias in the form of a near 4% house edge. We’re talking about a good old fashioned physical toss of coin.

A recent article in the Scientific American has shattered this widely held belief that a coin toss is completely fair.

The article details a study carried out by František Bartoš, a Ph.D. at the University of Amsterdam, in which the outcome of 350,757 tosses (completed by a team of 47 volunteers) resulted in a 50.8% bias to the side facing up at the start of the toss. According to statisticians the data set is way too large for the result to be a fluke. Put another way, the result is statistically conclusive.

“We can be quite sure there is a bias in coin flips after this data set”  .. František Bartoš

This outcome is supported by a 2007 physics study carried out at Stanford University. They took a different approach, calculating a flipped coins trajectory using laws of mechanics.

Turns out a flipped coin doesn’t spin uniformly on a horizontal axis. Rather, the laws of mechanics have it wobbling slightly on its axis of rotation. This in turn means it will spend marginally more time with its starting up-side on top.

Their calculations concluded a 51% bias to the starting up side – a similar result to Bartoš findings.

So next time you’re using a coin toss to settle a disagreement, if you get the option, always pick the starting up-side and you’ll have a slight edge. And if you’re giving your opponent the pick of heads or tails be sure to hide the starting up-side. This way the bias disappears altogether.