What does ${50}\choose{4}$ mean in statistics? I have a test tomorrow in statistics and was wondering what the following means?
$$\binom{50}{4}$$
My professor along with most of my classmates have a calculator they can just plug that into. The professor never went into depth on what it means and how to figure it out with a normal ti-30.
Thanks for your time. 
 A: ${50}\choose{4}$ is read "$50$ choose $4$" and is the number of ways to choose $4$ things from $50$ things, where order doesn't matter. Most graphing calculators will have a button for this, if not there is the formula:
$${{n}\choose{k}} =\frac{n!}{k!(n-k)!}$$
where $n!=n\times (n-1)\times\cdots\times 1$.
A: Binomial Coefficient or Combination
Wikipedia words it quite nicely: 

$\binom{n}{k}$ is read as "$n$ choose $k$", because there are $\binom{n}{k}$ ways to choose $k$ elements, disregarding their order, from a set of $n$ elements.

I'd bet that most calculators use this efficient method,
$$\binom{n}{k} = \prod\limits_{i=1}^k \frac{n+1-i}{i}$$
So for your example, we have
$$\binom{50}{4} = \prod\limits_{i=1}^4 \frac{51-i}{i}=\frac{50\cdot 49\cdot 48\cdot 47}{1\cdot 2\cdot 3\cdot 4}=230,300$$
Which means that there are 230,300 ways to choose 4 elements from a set of 50 elements, disregarding their order. The more common but less efficient method of computing this is
$$\binom{n}{k} = \frac{n!}{k!(n-k)!}\quad\mbox{for}\quad 0\leq k\leq n$$
So for your example, we have
$$\binom{50}{4} = \frac{50!}{4!46!}=\frac{50\cdot 49\cdot 48\cdot 47\cdot 46!}{1\cdot 2\cdot 3\cdot 4\cdot 46!}=230,300$$
Have a look at Binomial Coefficient, Combination and Factorial for a better understanding.
