Posted by Moot on 11/10/06 21:57

Kevin Williamson wrote:
> Hi,
>
> I've written a lottery programme in PHP that selects 10 winners a week
> from approximately 18,000 shares.
>
> The problem I have is that of the ten winners one week the same share
> number was drawn the next week and an adjacent share number the
> following week (that just happened to be owned by the same member)
> The program is now being looked at suspisciously.
>
> The main core of the random draw simply puts all the current share
> numbers into an array ($currWinner), draws a random number using the
> rand function between 1 and the number of shares ($lottshares) and
> returns the share id number.
>
> for the random element ofthe program I use:
>
> $currWinner = rand(1,$lottShares);
>
> is this random enough? or is it possible that it needs seeding (although
> I don't totally understand seeding?
>
>
> Any help will be appreciated.
>
> thanks
> Kevin


While (all things being random) it isn't technically impossible, it is
suspicious. Aside from the PHP version seeding issue Jerry mentioned,
you may want to just run a big simulation, running a few thousand
"drawings", then get the number of occurances of each share.

If it looks like a few numbers start jumping out more often than
others, you may have a problem. Ideally, in a truely random system,
when run for a sufficiently large set, all shares should have
relatively the same number of occurances.

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