Is it possible to upload pictures or excel sheets here?
I work out that with current returns you would need to invest €150,385 to return 100% chance of €100 prize (you would also get 36 x €50).
Where I struggle is this - if you toss a coin twice you have a 100% chance of throwing a head. But you probably wont - does that make sense? So can I count on the €100 prize or is this where "confidence" comes in. Can you send me you email address somehow Ill show you some workings.
Yes, there is an average number of wins that you can calculate, and this has the highest chance of coming up (although the odds are certainly not 100%). The confidence level falls off either side of this. You can sum the odds for each individual number of wins to get the odds of your returns falling in any given interval.
Re: 100% odds -- you're adding the 50% odds of a head on each of two tosses to get 100%. Odds don't sum like that -- no matter how many times you toss the coin there is always a slim chance that you will
never throw a head, so the odds can never be 100%. Perhaps counterintuitively, what you need to consider is the odds of
not throwing a tail. Think about it like this: suppose I toss a coin ten times and want to know the odds of getting at least one head; I could get a head on the first toss with 50% probability, or on the second, or on both first and second -- when you think of all the different ways that you could toss at least one head over ten tosses, it gets complicated.
But, there's only one way of getting
zero heads -- by getting tails on the first toss,
and tails on the second,
and tails on the third, and so on. When you want to combine the odds of a particular outcome in
each of a series of independent trials, you simply multiply them. So for a coin toss you have a 1/2 chance of getting tails on the first toss, 1/4 of on first
and second, 1/8 on first
and second
and third, and so on. The odds are 1/(2^
n) after
n tosses. And, of course, since if you don't throw all tails you must throw at least one head, the odds of that are 1 - (1/(2^
n)). So, for example, the odds of at least one head after two tosses is 3/4 or 75%.
For the odds of winning any given number of coin tosses, say
m wins out of
n tosses we have to resort to slightly more complicated calculations using the binomial coefficient. For prize bond wins the approach is the same, it's just there is a very much greater number of trials -- hundreds of thousands per year -- and your odds in each independent trial depend on what percentage of the total issued prize bonds you hold. Another complication is that the binomial coefficient calculation involves factorials, and for the prize bond calculation they are of very high degree -- higher than a spreadsheet is typically able to handle (e.g. Excel blows up above 10^309 ~ 170!). I found I had to use Stirling's approximation, which allows you to work in logs.
But if we're only interested in averages, that's much simpler -- just calculate the percentage return delivered by each category of prize, that is, the prize amount multiplied by the number of prizes divided by the €1.9b total fund. I get:
€1m -- 0.3158%
€20k -- 0.0484%
€1k -- 0.0137%
€100 -- 0.1368%
€50 -- 1.0853%
So just start at the bottom and add percentages for whichever prizes you want to include to get the average return -- the bottom two sum to 1.22%.
I'm open to correction. You can mail me your workings at
s3923-fb2@yahoo.ie. If required I can stick up a shared Google spreadsheet that we both can edit. Independent validation would be welcome.