Introduction To Probability

8.1 Introduction to Probability – Cheat Sheet

1. Probability scale

• 0 = impossible event
• 0.5 = equally likely to happen or not
• 1 = certain event

2. Probability notation

• P(A) = probability of event A occurring
• P(A’) = probability of event A not occurring
• P(A’) = 1 – P(A)

3. Formula for a single event
P(A) = (Number of favourable outcomes) / (Total number of possible outcomes)

4. Complementary probability

• P(A’) = 1 – P(A)
  Example:
  P(B) = 0.8
  P(B’) = 1 – 0.8 = 0.2

5. Probability representation

• Fraction: 3/4
• Decimal: 0.75
• Percentage: 75%

6. Using tables, graphs, or Venn diagrams

• From a table: P(A) = (frequency in A) / (total frequency)
• From a Venn diagram: P(A) = (number in A) / (total outcomes)
• P(A’) = (number not in A) / (total outcomes)