🎲Intro to Probability Unit 2 – Probability Axioms and Properties

Key Concepts

• Probability is a measure of the likelihood of an event occurring, expressed as a number between 0 and 1
• Sample space is the set of all possible outcomes of an experiment or random process
• Events are subsets of the sample space, representing specific outcomes or combinations of outcomes
• Probability axioms provide the foundation for calculating probabilities and ensure consistency in probability theory
• Properties of probability, such as the addition rule and multiplication rule, allow for the calculation of probabilities for complex events
• Conditional probability is the probability of an event occurring given that another event has already occurred
• Independence of events means that the occurrence of one event does not affect the probability of another event occurring
• Mutually exclusive events cannot occur simultaneously, and their probabilities add up to 1

Probability Axioms

• Non-negativity axiom states that the probability of any event A is greater than or equal to 0, denoted as $P(A) \geq 0$
• Normalization axiom states that the probability of the entire sample space S is equal to 1, denoted as $P(S) = 1$
• Additivity axiom states that for any two mutually exclusive events A and B, the probability of their union is the sum of their individual probabilities, denoted as $P(A \cup B) = P(A) + P(B)$
• Consequences of the axioms include:
  • The probability of the empty set (impossible event) is 0, denoted as $P(\emptyset) = 0$
  • The probability of the complement of an event A is 1 minus the probability of A, denoted as $P(A^c) = 1 - P(A)$
• The axioms ensure that probabilities are consistent and well-defined, preventing contradictions or paradoxes in probability calculations

Sample Spaces and Events

• A sample space is the set of all possible outcomes of a random experiment or process, usually denoted by S
• An event is a subset of the sample space, representing a specific outcome or a combination of outcomes
• Examples of sample spaces include:
  • Tossing a coin (heads, tails)
  • Rolling a six-sided die (1, 2, 3, 4, 5, 6)
• Events can be simple or compound:
  • Simple events consist of a single outcome (getting a head on a coin toss)
  • Compound events are combinations of simple events (getting an even number on a die roll)
• The complement of an event A, denoted as A^c, is the set of all outcomes in the sample space that are not in A
• The union of two events A and B, denoted as $A \cup B$, is the set of all outcomes that are in either A or B, or both
• The intersection of two events A and B, denoted as $A \cap B$, is the set of all outcomes that are in both A and B

Properties of Probability

• The addition rule states that for any two events A and B, the probability of their union is the sum of their individual probabilities minus the probability of their intersection, denoted as $P(A \cup B) = P(A) + P(B) - P(A \cap B)$
  • For mutually exclusive events, the intersection term is 0, simplifying the addition rule to $P(A \cup B) = P(A) + P(B)$
• The multiplication rule states that for any two events A and B, the probability of their intersection is the product of the probability of A and the conditional probability of B given A, denoted as $P(A \cap B) = P(A) \cdot P(B|A)$
  • For independent events, the conditional probability term simplifies to the unconditional probability, resulting in $P(A \cap B) = P(A) \cdot P(B)$
• The law of total probability states that for a partition of the sample space into mutually exclusive and exhaustive events $A_1, A_2, ..., A_n$, the probability of an event B can be calculated as $P(B) = \sum_{i=1}^n P(A_i) \cdot P(B|A_i)$
• Bayes' theorem allows for the calculation of conditional probabilities in the reverse direction, stating that $P(A|B) = \frac{P(B|A) \cdot P(A)}{P(B)}$

Calculating Probabilities

• To calculate the probability of an event A, determine the number of favorable outcomes (outcomes in A) and divide it by the total number of possible outcomes in the sample space
• For equally likely outcomes, the probability of an event A is given by $P(A) = \frac{|A|}{|S|}$, where $|A|$ is the number of outcomes in A and $|S|$ is the total number of outcomes in the sample space
• When dealing with continuous random variables, probabilities are calculated using probability density functions (PDFs) or cumulative distribution functions (CDFs)
  • The probability of a continuous random variable X falling within an interval $[a, b]$ is given by $P(a \leq X \leq b) = \int_a^b f(x) dx$, where $f(x)$ is the PDF of X
• Conditional probabilities can be calculated using the multiplication rule, $P(B|A) = \frac{P(A \cap B)}{P(A)}$, where $P(A) > 0$
• Independent events have the property that $P(A \cap B) = P(A) \cdot P(B)$, simplifying probability calculations

Real-World Applications

• Probability theory is used in various fields, such as:
  • Finance (portfolio optimization, risk assessment)
  • Insurance (calculating premiums, estimating claim frequencies)
  • Medicine (diagnostic testing, treatment effectiveness)
  • Machine learning (classification, regression, clustering)
• Example: In medical testing, the sensitivity and specificity of a test can be used to calculate the probability of a patient having a disease given a positive or negative test result using Bayes' theorem
• Probability is essential in decision-making under uncertainty, allowing for the quantification and comparison of different outcomes and their likelihoods
• Probabilistic models help in understanding and predicting complex systems, such as weather patterns, stock market behavior, and social networks

Common Misconceptions

• Gambler's fallacy is the belief that past events influence future independent events (thinking that a coin is more likely to land on heads after a series of tails)
• Confusion between conditional probability and joint probability, where $P(A|B)$ is mistakenly treated as $P(A \cap B)$
• Misinterpreting the meaning of independence, assuming that two events are independent when they are actually dependent or conditionally independent
• Misapplying the law of large numbers, expecting small samples to perfectly represent the population characteristics
• Misunderstanding the concept of randomness, attributing patterns or meaning to random events or sequences

Practice Problems

1. A fair six-sided die is rolled. What is the probability of getting an even number?
2. Two cards are drawn from a standard 52-card deck without replacement. What is the probability of getting two aces?
3. A bag contains 4 red balls and 6 blue balls. Two balls are drawn at random without replacement. What is the probability of drawing two balls of the same color?
4. In a certain population, 60% of people have brown eyes, and 10% have both brown eyes and are left-handed. If a person is selected at random and is found to be left-handed, what is the probability that they have brown eyes?
5. A fair coin is tossed three times. What is the probability of getting exactly two heads?