Maths question...probability

henrygregory

There are 1.2 billion people on facebook and on average have 229 "friends. What is the probability of you finding someone you know on a site such as this?

http://app.thefacesoffacebook.com/


I would be interested to know the answer, and also how you managed to calculate it!

danthemoneysavingman

someone specific or just anyone you know, and how well do you need to know them for that to count?

and how many ppl do you know?

WTFH

OK, so are you saying that in real life, the average person has 229 friends?

If so...
Let's say the global population is 7.2 billion (it's not far off that).
That means 1 person in every 6 is on facebook. (7.2/1.2 = 6)

The probability that anyone in the world is on facebook is 1/6, i.e. 16.67%

That means, of the 229 people you know, it's likely that 38 of them (229/6) are on Facebook.

WTFH

...but my answer doesn't tell you the probability of you finding them on Facebook. That's when you go into facebook security settings, and how you plan to search for them. Also, it's dependent on where your friends are, internet capabilities in that part of the world, their age, their IT literacy, etc, etc.

So, my answer is not to your question, but to the simpler one:
What is the probability that someone is on Facebook?

henrygregory

Goodness, your brains are both larger than mine!

Thanks for your input.@danthemoneysavingman, I would say not someone specific, just someone on your friends list, given that most users have on average 229 friends.

WTFH

henrygregory wrote: »

I would say not someone specific, just someone on your friends list, given that most users have on average 229 friends.

OK, if someone is on your friends list on Facebook, then the probability that they are on Facebook is 1.

But the probability that someone you know in the "real world" is on Facebook is 1/6. Doesn't matter how many people you know, or how well you know them.

paddyrg

Also, remember facebook isn't a random sampling of 1/6 of the population, but is very concentrated around certain demographics. If you know someone from real life, they are likely to be in a similar demographic to you than a peasant farmer in rural Burma. You may have shared friends or interests which made them a friend in the first place, and those shared friends or interests may have taken them onto facebook.

Personally, I can't stand facebook - happily properly fully deleted my profile and my life has not suffered in the least for it.

Mankysteve

Easy if your just picking names out of hat and which one likely to actually be your friend. (1.2 *10^9)/266

1 in every 4511278.2 should your friend, that presuming you return your friends name you picked out back into the hat. If you leave your friends name out the odd get worse for each pull and I can't do the maths for that.


but as you don't search for people on facebook that way the odds are very difficult/nearly impossible to calculate.

Mankysteve

WTFH wrote: »

But the probability that someone you know in the "real world" is on Facebook is 1/6. Doesn't matter how many people you know, or how well you know them.

It's not going to be 1/6 while a younger person for instance is going be more likely to have friends on facebook than say a 70 year pensioner and there more lilkly to have to friends on facebook than someone who lives in the third world.

WTFH

Mankysteve wrote: »

It's not going to be 1/6 while a younger person for instance is going be more likely to have friends on facebook than say a 70 year pensioner and there more lilkly to have to friends on facebook than someone who lives in the third world.

Hence my follow up post.

Also, it's possible to have friends who are not the same age as you or live in the same part of the world. So, it's not that dependent on the age of the person asking the question, more on how well they get on with others!

rmg1

Wouldn't it just be 1.2 billion / 229?
That would give you .........
1 in 52401746.724890829694323144104803 (according to my calculator anyway).
Then you've got the argument about which billion to use (US/UK) which could start a whole new debate.

I do agree with the other regarding demographics, population centres, mutual interest, etc, etc which does have the potential to skew the odds in a fairly major way.