Calculate Basic Probability — Determine probability of simple events.

The formal study of probability arose not from abstract mathematical curiosity but from deeply practical concerns—games of chance, actuarial reasoning, and the desire to make rational decisions under conditions of uncertainty. Long before anyone wrote a formula, civilizations from Mesopotamia to Rome played dice games, intuitively sensing that some outcomes were more likely than others. The transition from intuition to rigorous theory, however, required centuries of intellectual development spanning correspondence between gamblers and mathematicians, breakthroughs in combinatorics, and ultimately the axiomatization of the entire field. Understanding this history clarifies why basic probability is defined the way it is and why its formalism has proven so resilient across disciplines as varied as medicine, genetics, and data science.

Despite these sophisticated developments, the essential question remains deceptively simple: given a well-defined experiment, how do we assign a number between 0 and 1 to each possible outcome? Mastering this question—specifically for simple events with equally likely outcomes—is the gateway to every subsequent topic in probability and statistics, from conditional probability to hypothesis testing. For the TEAS examination, a confident command of these fundamentals is essential.

Before computing any probability, one must establish a precise vocabulary. A random experiment is any process whose outcome cannot be predicted with certainty—rolling a die, drawing a card, or selecting a patient record at random. The sample space (denoted S) is the set of all possible outcomes of that experiment, and an event is any subset of the sample space. A simple event consists of exactly one outcome, whereas a compound event consists of two or more outcomes. These definitions, while elementary, must be internalized because errors in probability almost always trace back to misidentifying the sample space or the event of interest.

The diagram above illustrates the most fundamental scenario in basic probability. The sample space S is enclosed in the outer boundary, and each individual outcome occupies its own cell. Because a fair die grants each face equal likelihood, the probability mass is distributed uniformly: each outcome receives exactly 1/6 of the total probability. The glowing cell for outcome 4 visually isolates the simple event E = {4}, making it clear that the probability of this event equals the ratio of one favorable outcome to six total outcomes. This visual pattern generalizes: for any experiment with n equally likely outcomes and a simple event comprising one specific outcome, P(E) = 1/n.

The classical definition of probability, attributed to Laplace, provides the formula most directly applicable to the TEAS. It applies whenever the sample space is finite and all outcomes are equally likely—conditions satisfied by dice, coins, well-shuffled cards, and random selections from uniform populations.

Note the critical precondition: the classical formula requires equally likely outcomes. If outcomes are not equally likely—for example, drawing from a weighted die or a biased coin—one must instead rely on empirical (frequentist) probability, defined as the limit of relative frequency over repeated trials. For the TEAS, most probability questions involve fair coins, standard dice, well-shuffled decks, or uniformly random selections, so the classical formula is your primary tool. However, always verify the equal-likelihood assumption before applying it.

Probability values span a continuous spectrum from 0 (impossible) to 1 (certain). Developing intuition for where an event falls on this scale is just as important as calculating the exact number. The diagram below provides a visual reference that maps common events to their positions on the probability number line, reinforcing the relationship between fractions, decimals, and percentages—all interchangeable representations of the same probability.

Consider the following problem: A standard 52-card deck is well shuffled and a single card is drawn at random. What is the probability that the card drawn is a king?

The classical probability formula is elegant and powerful within its domain, but it carries assumptions that limit its applicability. Recognizing these boundaries is essential for graduate-level reasoning, because real-world data rarely emerge from perfectly symmetric experiments. The table below contrasts the strengths and limitations of this approach.

Simple-event probability serves as the entry point into a much richer mathematical framework. Once you are comfortable computing P(E) for a single event, the natural next questions involve combining events (union and intersection), incorporating new information (conditional probability), and reasoning backwards from observed data to underlying causes (Bayes' theorem). The table below maps the concepts covered in this lesson to their advanced counterparts, giving you a roadmap for subsequent study.

For the TEAS, mastery of simple-event probability and the complement rule is sufficient. However, recognizing that these concepts form the foundation of clinical biostatistics, epidemiological risk assessment, and evidence-based medicine underscores why the exam tests them so deliberately. The ability to correctly define a sample space, enumerate favorable outcomes, and compute a ratio is the indispensable first step toward every quantitative reasoning task you will encounter in graduate health science programs.