Counting Outcomes

Counting Outcomes Basics

  • Counting outcomes refers to determining the number of possible outcomes in a random experiment or situation.
  • Counting outcomes is important in probability because the total number of outcomes often forms the denominator when calculating the probability of a particular event.

Fundamental Counting Principle

  • The Fundamental Counting Principle can be used to find the number of possible outcomes when there are multiple stages or parts to a problem.
  • If an event can happen in m ways and a second event can happen in n ways, the total number of ways both events can happen is m × n.
  • For example, if you have 3 shirts and 4 trousers, you can make 3 x 4 = 12 different outfits.

Use of Tree Diagrams

  • A tree diagram is a useful tool for counting outcomes in more complex situations. Each branch of the tree represents a possible outcome.
  • To count the total number of outcomes, simply count the number of ‘ends’ to the tree diagram.

Combinations and Permutations

  • Permutations refer to the number of ways of selecting items where the order of selection matters.
  • Combinations refer to the number of ways of selecting items where the order does not matter.
  • The number of combinations of r items from a set of n unique items can be found using the formula: nCr = n! / r!(n - r)!, where ! represents factorial.
  • The number of permutations of r items from a set of n unique items can be found using the formula: nPr = n! / (n - r)!.

Factorials

  • A factorial of a number is the product of that number and all the positive integers less than it.
  • Factorial is represented by an exclamation mark (!).
  • For example, 5! = 5 × 4 × 3 × 2 × 1 = 120.

Inclusion-Exclusion Principle

  • The Inclusion-Exclusion principle is used for counting the number of outcomes when events have overlap.
  • If two events, A and B, have overlap then the total number of outcomes is: n(A) + n(B) - n(A ∩ B), where n(A ∩ B) is the number of outcomes that belong to both A and B.
  • The principle can be extended for more than two events.

Use of Grids and Lists

  • For simpler problems, you might also use grids or lists to count outcomes.
  • For example, if two dice are thrown, you could use a 6x6 grid to represent all 36 possible outcomes.