## The Economist ~ A very short article about maths.

*Day 2,221, 10:59*• Published in United Kingdom •

*by*

Iain Keers

Dear friends,

A few brief notes on probability:

**The eight sided dice**

Say you have an eight sided dice. The dice is weighted so every side has an equal probability of showing up.

The odds of any particular side showing up is one in eight.

**The combi bet**

Now say you have three of these dice. You roll all 3. The chances of three of the same side showing up (4-4-4 or 5-5-5 or whatever) is 1 in 8^3 or 1 in 512.

**Independent probability**

Now say instead you have just one dice, but you keep rolling it. The chance of rolling a 5 is 1/8 each time, so you should expect on average to roll a 5 once for every eight attempts, though of course you could roll it a million times and never actually get a 5.

This is how the jackpot would work in the event it is an even chance of landing on it each time. It isn't a 1/512 chance.

To put it more simply, take coin flipping. You can flip a coin a hundred times, and the chance of getting a hundred heads in a row is equal to the chance of getting any other combination. Because each time it is a 50/50 chance.

This belief that the odds "stack" is one of the most common misconceptions in probability.

Sure, the chance of the first three goes you have being jackpots seems very low, but as you are effectively tossing dice or flipping a coin each time, the chance is equal to you hitting it with any other combination (if the "toss" is unweighted). Meaning you would normally get all three jackpots with an average frequency of 24 attempts, which is about 51.6 gold.

**Admins don't believe in maths**

Unfortunately it is highly unlikely all of the different outcomes have equal "weight". Having played the wheel itself I got the first jackpot on my third attempt, the second on my twentieth or so roll. I guess that the jackpot becomes increasingly harder to "hit" as you click. If I was the admin I'd make the median payout for all three at around the 350-400g mark. Most people would give up before then, but that way enough people would succeed to draw others in.

To be honest I'm not sure whether it's random at all, the whole thing could be scripted.

But anyway it just annoys me when people start talking about independent probabilities like they're conditional probabilities.

Iain out

Ps. it's been 10 years (gosh really!) since I've had to do any maths in a serious way, so if I've made any mistakes lemme know and I'll fix em.

Comments

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Comment deletedeRepublik keeps eating my massive replies. I think that this could be said better. I'll go for TLDR this time and maybe try t post the detail again later.

1/512 is indeed wrong, that is the probability of rolling three 8's in a row which is a 0.2% chance. You don't need to roll them all in a row AND you can hit free roll.

If you take into account all of the possible ways of winning then you'd expect to win on average after 21 rolls rather than 512. Draw a tree diagram.

Of course it's probally not a fair spinner.

No.

Biased dice! \o/

Miau

The probability of the outcomes having equal weights is infinity^-1.

I believe this article shows us the best way to use the power spin:

http://www.erepublik.com/sr/article/-power-spin-ftfy-2354053/1/20

v

"if the "toss" is unweighted" -ofc it is weighted

I doubt there is anything legally binding here in terms of the odds. The bias must be ridiculously high and I doubt they have considered any median payout. There is probably some gold intake quota along with player quota and once they are filled, the script will randomly pick a person, could be a player spinning for the first time or someone who spends the 500+th gold on this. the house never loses

Comment deletedHmmm, math in english must be translated

Or probability changed since I went to school.

Voted and shouted ! o/

Voted and shouted !

*-*

odds are weighted the same way the bazooka parts odds are. good luck wasting your G. I will be there selling it for you when it will go ridiculously high ; )

I don't think running out of money is a major concern for me...

It was a collective "your", and in average (since we are doing some math here) it may be a concern. In fact I have already seen some playing till running out of gold stories on my wall.

For you personally, have fun spinning the wheel!

"Meaning you would normally get all three jackpots with an average frequency of 24 attempts,"

21 attempts. You forgot about the "spin again" reward, which alters the probabilities a bit, and in fact they're conditional in some sense.

1/8 is the chance of getting a jackpot, and 1/8 is a chance of rolling again, therefore the probability of finding a jackpot is equal to a series:

(1/ + (1/^2 + (1/^3 + ... + (1/^n as n goes to infinity. It's 1/7.

"which is about 51.6 gold." Also wrong, even with 1/8 you cannot calculate it from the following formula: (1.0 + 3.3) / 2 * 24, why?

Because the cost function is not linear, for example, if f(x) is the cost function and its argument is the number of trials:

f(23) = 48,3,

f(24) = 51.6,

f(25) = 56.25

avg(f(23),f(25)) = 52.275.

To make matters worse, this isn't even a normal distribution, so I have no idea what approach to take except bruteforcing it. So here is it, an algorithm of simulating the power spin game written in java http://pastebin.com/zTUZ94tN easily adaptable to other programming languages. Anyway, the results are as follows:

Average cost after 10000000 tries: 48.3028846

Average number of tries: 21.0010793

So on average, with unweighted roulette wheel, 48.3 gold and 21 trials are needed to get the 500g reward, which is obviously unreal, and such great odds make it look pretty phishy. So instead I think I'm gonna grab a combat stash for myself, which is apparently new (because otherwise, why would anyone add a "new" label on it?), and its price has been lowered from a whopping 87eur to just 9.9eur! Or not.

it's weighted for sure. if the weight of each possible event is preditermined and static, or it is scripted and dynamic to satisfy some specific conditions is to be determined.

Too much math, you never win against the house should suffice xD

The chance of three 8-sided dice getting the same value is 8 in 512 (512 combinations but 8 different combinations that win) aka 1 in 64.

*tips hat*

Comment deletedHa, well said

Try and have fun and stop over analyzing everything, I can imagine you would be a loads of laughs standing next to a fruit machine

Laughing at the people who still use fruit machines in 2013 maybe

I haven't in about twenty years.

Meantime in Ireland...

http://www.erepublik.com/en/article/so-what-039-s-up-1-2354442/1/20

I could be wrong about this

I thought most coins, American at least, were actually weighted a bit more on the heads side. That would increase the odds of a tails result, wouldn't it, as the extra weight would tend to land on the bottom?