Dieter's Blog

Coin flips are lousy

Coin flips are lousy.

It's a useful metaphor to simplify a binary decision to a common object; "heads or tails?" is a ubiquitous phrase.

Coins were not designed to serve as dice. One side is heavier than the other (or "the center of mass is not coincident with the physical center of the object").

Persi Diaconis has done considerable research into this.


Numberphile - Persi Diaconis on coin tosses

That's fascinating stuff, documenting the inherent bias in a deterministic coin toss. There are much longer lectures and presentations on Youtube as well.

This wholly ignores the more real problem: some people can and do add further bias to the process. You can probably learn it yourself in an afternoon. It involves the set (starting heads up or tails up), making a consistent toss (surprisingly easy), and then repetition. Once you have a consistent toss, you can further bias the outcome with a controlled catch.

If you're not convinced there is a bias, do an experiment.
I'm assuming you're in the US for this.

Select a coin.
Prepare to flip it.


What to do about it

Don't toss coins. Instead of heads or tails, ask even or odd - roll some dice. Ask red or black - pick a card (any card).

You probably have some common dice or playing cards around, and they're considerably less likely to show a bias.