Comment by henrikschroder | Hacker News Reader

Comment by henrikschroder | Hacker News Reader

Parent Full thread

Oh yes. Our intuition is that random is fair and uniform, but it's absolutely not.

I was bit by another random quirk in World of Warcraft. There was a long-running achievement called "What a Strange Long Journey it has Been", which took at least a year in real-time to complete, and you had to actively play during each of the ten or so in-game festivals and holidays, and do some quests and tasks during each. If you missed a festival, you had to wait another year to do it again, so if you were aiming for the achievement, you really wanted to do it in one go.

During their version of Valentine's, each player got a bag of heart-shaped candy, and the task you had to complete was to pull out at least one each of the eight different heart candies. But you could only pull a piece of candy once every hour or so, each time you pulled a piece you had a 1/8 chance to get a certain piece, but the holiday was time-limited, so you had about two weeks to complete it.

Sounds easy and fair, right? 1/8 chance, get all eight pieces, two whole weeks, easy! Except that there was one piece that I just never got. The piece that said "I LOVE YOU!". And as the time went by, I got more and more frantic, logged in more often so as not to miss any opportunity to pull a piece of candy, but no luck.

So, I did a quick bit of math. You can pull one piece every hour for two weeks, but sleeping, not playing, missing days etc, meant that I effectively pulled ~100 pieces. The chance of missing a certain piece is 7/8, so missing a certain piece 100 times in a row is (7/8)^100, which is roughly a little bit more than one in a million.

With ten million players all doing the same thing, there's going to be a number of them that will hit that "one in a million" chance, which means that whoever designed that part of the achievement didn't do their homework, didn't do the math. Because intuition tells you that 1/8 chance is plenty! Hundreds of tries, of course everyone will get all pieces! Except proper math tells you otherwise, and I was one of the "lucky" outliers.

(Later, they apologized and retroactively removed that part of the achievement, so I got my purple nether-drake mount without having to wait a year extra.)

So, I did a quick bit of math. You can pull one piece every hour for two weeks, but sleeping, not playing, missing days etc, meant that I effectively pulled ~100 pieces. The chance of missing a certain piece is 7/8, so missing a certain piece 100 times in a row is (7/8)^100, which is roughly a little bit more than one in a million.

The chance of failure would be 8 times that much because there are 8 pieces you could miss, which comes to about 12.7 per million.

You couldn't possibly miss all 8 pieces. As soon as you make one attempt you'll get one piece.

Henrik's math calculated the chance of missing the "I love you" piece each time. The chance of missing any one other piece each time is the same.

Yes, but they're not independent. Given that you missed piece X, the probability that any one piece it's piece Y it's 1/7, not 1/8, so the chance of missing both X and Y it's lower than the product of their individual probabilities.

'missing each of 8 pieces individually' is not 'missing a certain piece'.

This is an example of a problem called the Coupon Collector problem (https://en.wikipedia.org/wiki/Coupon_collector%27s_problem). Short version is that the expected value of number of pulls you'll need to collect all of the coupons (candy in this case) is nln(n), or about 16 pulls in this case. Of course some people are just going to be unlucky (as you demonstrated).

> Oh yes. Our intuition is that random is fair and uniform, but it's absolutely not.

But a uniform distribution is random, isn't it? This sort of phrasing is rampant in this thread, and I'm confused by it. How can a uniform distribution be considered non-random, or "less" random than another distribution?

I can't tell if you're trolling or not. In case you aren't, a uniform distribution isn't "uniform" in the sense of the above quotation. You're right that it's not less random, but someone looking at draws from a uniform distribution would normally describe it as "clumpy"; hence the article.

A distribution is the probability of various values before the random selection takes place. Uniformity is what we do or don't see afterward.

The results of a uniform distribution, applied randomly a large number of times are less uniform than intuition predicts.

If you had bothered to read the article, your questions would have been answered.

Randomness is about having a uniform probability of results, but that does not translate into a uniform distribution of results, nor can it, because of the de-correlation between random results. Specifically, the chance of getting the same result multiple times is non-zero, and actually can be fairly high with a lot of samples, whereas the chance of duplicate results in a uniform distribution is zero.

Read the article for more details and examples.

Randomness is about having a uniform probability of results

Well, you're still assuming the colloquial definition of "random," which implies "uniformly random." Of course, we can have a random process that is not uniformly random.

As a completer of the "what a long strange trip its been" ( http://www.wowhead.com/achievement=2144/what-a-long-strange-... ), fuck Halloween.

A little randomness is good, it keeps the experience from being monotonous, but there is a balance between uncontrollable and unpredictable annoyance and quirky interesting outlier behavior. A lot of why WoW did so well was because it often walked the fine line, and it did it everywhere. In that game crafting had percent chances of giving skillups, you had to level weapon skills with a percent chance per hit of getting a point, you had critical % chance, hit % chance, dodge %, parry %, block %, you had talents that gave you % procs for special effects. Tremendous amounts of RNG that most people never even noticed but kept them playing for so long in many respects.

Reply on news.ycombinator.com