The permutation formulaapplies to only two groups of predetermined size. Look for a specific reference to "order" being important.
x: Calculate the number of ways to choose r objects from a total of n objects, when the order in which the r objects is listed does or does not matter.
A special case of this labeling is when k = 2. That is that n items can only be in one of two groups and n1+ n2= n. In that case, we can let r = n1and n2= n - r. Since there are only two categories, we usually talk about choosing r items. Then (n - r) are notchosen. The general formula for labeling when k = 2 is called the combination formula (or binomial formula):
n! / (n - r)! * r!
Example:In a portfolio of eight stocks, we decide to sell three stocks. How many ways can we choose three of the eight to sell? The answer is 8! / (5! * 3!) = 56.
1.D: Common Probability Distributions
 
a: Explain a probability distribution.