Editing Guides/Probability (section)

=== I don't care when but ''k'' in ''n'' ===
where ''k'' is another natural number, less or equal to ''n''. In the example above we find 3 possible ways to get a series of colored marbles with one black and two white marbles (we don't really get to see it as we have to put the marbles back), each of them has the chance of (299/300)(299/300)(1/300), so the probability to get one of those series is 3 times that. Now remember, we had p = 1/300 and q = 299/300, so we can write the probability as
 3 * p^1 * q^2.
The general case, to make a long story short, looks like this
 ( (n!)/(k!(n-k)!) ) * p^k q^(n-k),
where the first term is the [http://en.wikipedia.org/wiki/Binomial_coefficient binomial coefficient] of ''n'' and ''k''.

We can now use the above formula to calculate what to expect in theory. In probability theory there's a so called [http://en.wikipedia.org/wiki/Expected_value expected value], that tells us what we should expect from an experiment "on average". The expected value while rolling a regular die is 3.5 by the way, even though you will never actually roll a 3.5 on a regular die. The expected value of the above experiment is
 n*p.
In our example with the marbles this means that if we made a new experiment in which we repeated the old experiment 300 times (series of 300 marbles), we can expect to see the black marble once on average in a series. In other words: if you have an item that has a chance to break of 1 in 10k, you should expect it to break in 10k uses/hits.

There's one more value to consider here and that's the [http://en.wikipedia.org/wiki/Variance variance]: it tells us how far spread out we should expect results from the expected value, think of it as a measurement to tell the extremely lucky/unlucky from the regular lucky/unlucky people. For our problem the variance is calculated to
 n * p * q.
Putting this to use in our series of 300 marbles example we see that the variance is q (as n*p=1), which means we shouldn't really be expecting to have series with more than 2 black marbles in them (0 is possible of course). However if we did series of 600, we'd expect 2 black marbles per series on average and as much as 4 and as low as 0.