How to make a drawing fair and transparent?

Simple, use publicly available data so everyone can verify who the winner is.

Let me back up and provide some context first. I’m running a twitter contest to give away free versions of Recipy every week (this week was the first one). See the post for full contest rule. Now that the first week is over, I need a way to randomly select a winner, and iterate every week. I could pick a winner randomly right by myself, but that could be criticized, and I may be playing favorite (not that I would).

So, here’s a simple method. You need two known facts:
– the list of participants, each one with a number, published by the end of the contest period, and ahead of the random number
– you need a random number only available by the end of the contest period (although since the number of participants can vary till the last minute, this is not critical)

First step is easy. And there are many ways to do the second. You could use lottery numbers, the price of a stock (although fairly random, not enough variation), or you could use numbers published by random.org. random.org happens to publish daily numbers. As the number is a bit big hugehumongus, I’ll just use the first 20 bits. Then translate to decimal (you can use many methods, here’s one: http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html). Then you use the remainder when divided by the number of entrants (modulo operation) plus one so you have number between 1 and your number of participants.

I might have lost you with a bit of geeky speak, so let’s illustrate with a concrete example using the first drawing.

we have 7 participants (no need to start big, the first ones are the ones with their fingers on the pulse of cool stuff):
1: @Jeyla1983
2: @t87
3: @ma_ttie
4: @lukasini
5: @seven
6: @AndreeBlixt
7: @MathieuLF

june 1st first 20 bits: 10000011110111010010101001001011000 (file here)
in decimal: 540114
remainder: 540114 mod 7 = 1
plus 1: 2

Our winner is #2, @t87 Congrats!

And may luck be with you next week, using june 8 number file. New entrants will get a number higher than 7, and all existing participants keep their numbers for an other chance every week. Yes, potentially, you could win more than one copy 🙂

Update: someone pointed out that “10000011110111010010101001001011000” is more than 20 bits! True indeed, thanks. However, the math was right and was using “10000011110111010010”.