Revision sheet: Introduction to Business Probability

📋 Course Outline

1. Random Experiments and Trials
2. Sample Spaces, Outcomes and Events
3. Mutually Exclusive and Exhaustive Events
4. Venn Diagrams and Event Relationships
5. Subjective and Frequentist Probability
6. Probability Axioms
7. Expected Value and Linearity

📖 1. Random Experiments and Trials

🔑 Key Concepts & Definitions

• Random experiment : A random experiment is a repeatable process where uncertainty remains and the set of possible outcomes is known.
• Trial : A trial is one single execution of a random experiment, producing one observed result.

📝 Essential Points

• A business example of a random experiment is the number of customers in a day for a store because it is uncertain, repeatable, and has known possible outcomes.
• In a random experiment, uncertainty comes from not knowing which outcome will occur before the trial happens.
• Opening the store every morning can be the random experiment, while the number of customers on a given day is the trial.
• In a business context, one trial can be one customer visit, one loan default, or one day of sales.

💡 Memory Hook

Experiment = the procedure; trial = the one run that happens.

📖 2. Sample Spaces, Outcomes and Events

🔑 Key Concepts & Definitions

• Sample space : The sample space is the collection of all possible outcomes of a random experiment.
• Outcome : An outcome is one specific result from the sample space.
• Event : An event is a set of outcomes that matches what matters for the question.

📝 Essential Points

• If you confuse sample space with what happens, you miss that it lists possibilities rather than observations.
• The sample space must be fixed before observing outcomes for the modeling to stay coherent.
• Events group together multiple outcomes that are relevant to the question.
• An event can include many outcomes, such as “a customer buys a coffee and a croissant.”
• The empty set can be an event because it represents an impossible event.

💡 Memory Hook

Event = a bundle of outcomes, not a single outcome.

📖 3. Mutually Exclusive and Exhaustive Events

🔑 Key Concepts & Definitions

• Mutually exclusive events : Mutually exclusive events are events that cannot occur at the same time in the same trial.
• Exhaustive events : Exhaustive events are events for which at least one must occur in every trial.

📝 Essential Points

• Mutual exclusivity means the overlap is impossible, so the events cannot both happen together.
• A business example of mutually exclusive events is paying cash or paying card, but not both.
• Mutual exclusivity is important because it allows adding probabilities directly without double-counting.
• Exhaustive events mean the set of considered events covers all possibilities so that something happens.
• Exhaustive events can be exhaustive without being mutually exclusive because they may overlap.
• Exhaustive events are useful because they ensure total probability accounts for everything (equals 1).

💡 Memory Hook

Mutually exclusive = no overlap; exhaustive = nothing left out.

📖 4. Venn Diagrams and Event Relationships

🔑 Key Concepts & Definitions

• Venn diagram : A Venn diagram is a picture that represents events as regions and shows overlaps to reason about event relationships.
• Event overlap : Event overlap is the shared region of two events, representing outcomes belonging to both events.

📝 Essential Points

• A Venn diagram helps you visualize relationships between events.
• Using overlaps in reasoning helps catch double-counting errors when events are not disjoint.
• A Venn diagram can become impractical when there are many events or high-dimensional relationships.
• The probability of an event is equal to the relative size of its region within the sample space.

💡 Memory Hook

Overlap region is where double-counting would happen.

📖 5. Subjective and Frequentist Probability

🔑 Key Concepts & Definitions

• Subjective probability : Subjective probability is a probability assignment based on a person’s information or beliefs.
• Frequentist (empirical) probability : Frequentist probability defines probability using long-run relative frequencies from repeated trials.

📝 Essential Points

• Two people may assign different probabilities to the same event because they may have different information or beliefs.
• Subjective probability is unavoidable in business situations involving new markets, strategic decisions, or rare events.
• Frequentist probability relies on repeating the process many times to estimate stability.
• Frequentist probability is hard to apply when the business event is essentially unique, like starting a new company.

💡 Memory Hook

Subjective = what you believe; frequentist = what you observe repeatedly.

📖 6. Probability Axioms

🔑 Key Concepts & Definitions

• Probability axioms : Probability axioms are foundational requirements that define a consistent mathematical measure of probability.
• Axiom of non-negativity : The axiom of non-negativity requires every probability to be at least 0.
• Axiom of normalization : The axiom of normalization requires the probability of the entire sample space to equal 1.

📝 Essential Points

• We need axioms instead of proving probability rules because they establish a consistent starting framework for probability.
• If probabilities did not follow axioms, they could become illogical, such as being larger than 1 or smaller than 0.
• Axioms connect intuition to mathematics by turning chance reasoning into constraints that can be used reliably.
• Non-negativity makes no sense because negative likelihood has no meaning for chance.
• If non-negativity failed, probability would no longer represent chance.
• Normalization expresses certainty because something must happen in every trial, so the sample space has probability 1.

💡 Memory Hook

Axioms stop probability from becoming negative or exceeding 1.

📖 7. Expected Value and Linearity

🔑 Key Concepts & Definitions

• Expected value : Expected value is the weighted-average value of a random variable across all possible outcomes.
• Linearity of expected value : Linearity of expected value states that the expected value of sums can be computed from simpler expectations.
• Weighted average : A weighted average multiplies each outcome by a weight (often its probability) and sums the results.

📝 Essential Points

• Expected value is often called a theoretical average because it is not based on a single observed sample.
• The expected value does not need to be an outcome that actually occurs in any one trial.
• In business, expected value can represent average profit, average cost, or average number of customers.
• Expected value calculations use weights where coefficients multiply each outcome and the weights sum to 1.
• When all outcomes are equally likely (e.g., a die), the expected value becomes a simple average where each weight is equal.
• Linearity is powerful because it simplifies computations and decision-making in economics or finance.

💡 Memory Hook

Expected value = average in theory, not necessarily a real observed result.

⚠️ Common Pitfalls & Confusions

1. Confusing the sample space with what actually happens leads to listing possibilities incorrectly.
2. Defining an event as a single outcome instead of a set breaks probability reasoning for “at least one” style questions.
3. Trying to add probabilities for overlapping (non-mutually exclusive) events causes double-counting and totals greater than 1.
4. Treating exhaustive events as automatically mutually exclusive leads to missing that overlaps can still allow coverage of all outcomes.
5. Using a Venn diagram without accounting for overlaps makes it easy to miscount outcomes.
6. Applying frequentist probability without enough repetitions can give unstable results.
7. Assuming expected value must be an outcome that occurs in the real world confuses “average in theory” with a specific trial result.

✅ Exam Checklist

1. Identify which business situation qualifies as a random experiment and explain what makes it random.
2. Distinguish a random experiment from one trial using a concrete execution-example.
3. List and describe the sample space as all possible outcomes, not what happens.
4. Explain how the sample space must be fixed before observing outcomes.
5. Define outcome and event, and justify why an event is treated as a set.
6. Recognize an event containing many outcomes and interpret the empty set as an impossible event.
7. Decide whether two events are mutually exclusive and state why that matters for adding probabilities.
8. Determine whether a collection of events is exhaustive and explain what exhaustive implies.
9. Explain why exhaustive events are useful for ensuring total probability coverage.
10. Use a Venn diagram to reason about event relationships and identify double-counting overlaps.
11. State when Venn diagrams become impractical and why.
12. Explain why subjective probabilities can differ between people.
13. Describe frequentist probability as long-run relative frequency and why repetition is needed.
14. Give an example where frequentist probability is hard to apply in business (unique event).