2.1 Basic probability concepts and rules

Probability theory forms the backbone of statistical inference, providing tools to quantify uncertainty and make predictions. This section introduces key concepts like sample spaces, events, and axioms that lay the groundwork for understanding more complex probabilistic ideas.

Building on these foundations, we explore essential rules for calculating probabilities, including addition and multiplication. These rules, along with concepts like conditional probability and independence, are crucial for solving real-world problems and making informed decisions based on data.

Probability and its Axioms

Defining Probability and Sample Space

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• Probability measures the likelihood of an event occurring numerically between 0 (impossible) and 1 (certain)
• Sample space encompasses all possible outcomes in a probability experiment
• Event represents a subset of the sample space
• Probability of the entire sample space always equals 1
• Probability of an impossible event always equals 0

Fundamental Probability Axioms

• For any event A, probability of A occurring plus probability of A not occurring equals 1: $[P(A)](https://www.fiveableKeyTerm:p(a)) + P(A') = 1$
• Probability of union of mutually exclusive events equals sum of individual probabilities: $P(A \cup B) = P(A) + P(B)$ (when A and B are mutually exclusive)
• Probabilities are non-negative: For any event A, $P(A) \geq 0$
• Axioms of probability form foundation for all probability calculations and statistical inference (hypothesis testing, confidence intervals)

Probability Rules: Addition and Multiplication

Addition Rule and Its Applications

• Addition rule for probability states $P(A \cup B) = P(A) + P(B) - P(A \cap B)$ for any two events A and B
• Applies to both mutually exclusive and non-mutually exclusive events
• Used to calculate probability of either event occurring (rolling a 6 or an even number on a die)
• Extends to more than two events using principle of inclusion-exclusion

Multiplication Rule and Conditional Probability

• Multiplication rule for independent events: $P(A \cap B) = P(A) \times P(B)$
• General multiplication rule for dependent events: $P(A \cap B) = P(A) \times P(B|A)$
• $P(B|A)$ represents conditional probability of B given A has occurred
• Applied in scenarios like drawing cards without replacement or genetic inheritance

Advanced Probability Concepts

• Law of total probability states $P(A) = P(A|B_1)P(B_1) + P(A|B_2)P(B_2) + ... + P(A|B_n)P(B_n)$, where B₁, B₂, ..., Bₙ form a partition of the sample space
• Bayes' theorem, derived from multiplication rule, calculates conditional probabilities: $[P(A|B)](https://www.fiveableKeyTerm:p(a|b)) = \frac{P(B|A) \times P(A)}{P(B)}$
• Used in medical diagnosis, spam filtering, and machine learning algorithms

Probability with Combinatorics

Fundamental Counting Principles

• Combinatorial methods involve counting techniques for determining number of ways events can occur
• Multiplication principle states if one event occurs in m ways, and another independent event in n ways, then two events occur together in m × n ways
• Applied in scenarios like choosing outfit combinations (3 shirts, 2 pants = 6 combinations)

Permutations and Combinations

• Permutations used when order matters, calculated as $P(n,r) = \frac{n!}{(n-r)!}$
• Combinations used when order doesn't matter, calculated as $C(n,r) = \frac{n!}{r!(n-r)!}$
• Permutations applied in ranking problems (top 3 finishers in a race)
• Combinations used in selection problems (choosing committee members)

Applications in Probability Calculations

• Binomial probability formula $P(X = k) = C(n,k) \times p^k \times (1-p)^{n-k}$ uses combinatorial methods
• Essential for calculating probabilities in complex scenarios (card games, lotteries, sampling problems)
• Applied in calculating probability of specific outcomes in repeated trials (flipping a coin 10 times and getting exactly 6 heads)

Independence in Probability

Defining and Testing Independence

• Events A and B are independent if occurrence of one doesn't affect probability of the other: $P(A|B) = P(A)$ and $P(B|A) = P(B)$
• For independent events, $P(A \cap B) = P(A) \times P(B)$ serves as both definition and test for independence
• Independence allows simplification of probability calculations in complex scenarios (repeated coin tosses, dice rolls)

Conditional Independence and Its Implications

• Conditional independence states events A and B are conditionally independent given C if $P(A|B,C) = P(A|C)$ and $P(B|A,C) = P(B|C)$
• Crucial in Bayesian networks and probabilistic graphical models
• Affects interpretation of relationships between variables in statistical analyses

Importance in Statistical Models and Inference

• Independence serves as crucial assumption in many statistical models and inference procedures
• Applied in Central Limit Theorem, allowing for normal approximation of sampling distributions
• Used in regression analysis, assuming independence of errors
• Recognizing non-independence crucial for choosing appropriate probability calculation methods (time series analysis, clustered data)