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What Is Probability?
Probability measures the likelihood of an event occurring, expressed as a number between 0 and 1 (or 0% to 100%). It is calculated as the number of favorable outcomes divided by the total number of possible outcomes. A probability of 0 means impossible; 1 means certain.
Probability vs Odds
Probability and odds express likelihood differently. Probability is favorable / total (e.g., 3/10 = 0.3). Odds are favorable / unfavorable (e.g., 3:7). To convert probability to odds: odds = p / (1-p). Odds are commonly used in gambling and medical research.
Basic Probability Rules
The complement rule: P(not A) = 1 - P(A). The addition rule for mutually exclusive events: P(A or B) = P(A) + P(B). The multiplication rule for independent events: P(A and B) = P(A) × P(B). These rules form the foundation of probability theory.
Applications of Probability
Probability is used in weather forecasting, insurance risk assessment, medical diagnosis, game theory, machine learning, quality control, and financial modeling. Understanding probability helps you make better decisions under uncertainty and evaluate risks in everyday life.
Frequently Asked Questions
Theoretical probability is calculated from known outcomes (e.g., a fair coin has 0.5 chance of heads). Experimental probability is determined by actual trials (e.g., flipping a coin 100 times and recording results). They converge as the number of trials increases.
No. Probability ranges from 0 to 1 (or 0% to 100%). If your calculation gives a value greater than 1, there is an error in your inputs — favorable outcomes cannot exceed total outcomes.
The complementary probability is the probability of an event NOT happening. It equals 1 minus the probability of the event. For example, if P(rain) = 0.3, then P(no rain) = 0.7.
For independent events happening together, multiply their probabilities. For either event happening, add their probabilities (subtracting the overlap for non-mutually exclusive events). This calculator handles single-event probability.