Question

Compute the expectation \(E(X)\) of the random variable \(X\) that counts the number of heads in four flips of a coin that lands heads with frequency \(1 / 3\).

Short answer

The expected number of heads is \(\frac{4}{3}\).

Step-by-step solution

Understand the Problem

We need to compute the expectation of the random variable \(X\), which counts the number of heads obtained in four flips of a coin. The coin lands heads with a probability of \(\frac{1}{3}\).

Identify the Distribution

The random variable \(X\) follows a binomial distribution since the problem involves repeated independent trials (coin flips). It counts the number of heads in a fixed number of flips (4), with a success probability \(p = \frac{1}{3}\).

Recall the Expectation of Binomial Distribution

For a binomial distribution with parameters \(n\) (number of trials) and \(p\) (probability of success), the expected value \(E(X)\) is given by the formula: \[E(X) = n \cdot p\]

Calculate the Expectation

Substitute \(n = 4\) and \(p = \frac{1}{3}\) into the formula: \[E(X) = 4 \times \frac{1}{3} = \frac{4}{3}\]

Confirm the Calculation

Re-evaluate the substitution and calculation to ensure no errors. The expected number of heads in four flips of the coin is \(\frac{4}{3}\).

Key concepts

Binomial Distribution

The binomial distribution is a foundational concept in probability and statistics, often used to model scenarios where there are fixed numbers of trials, each with two possible outcomes. These outcomes are commonly termed as "success" and "failure." Here, success could mean getting heads when flipping a coin.
The key properties of a binomial distribution include:

• **Number of trials (n):** This indicates how many times an experiment is conducted. In our case, a coin is flipped four times, so, n = 4.
• **Probability of success (p):** This is the chance of success in a single trial. If a coin has a \( \frac{1}{3} \) probability of landing heads, then p = \( \frac{1}{3} \).
• **Independence:** Each trial must be independent of each other, ensuring one trial's outcome does not affect another. Coin flips fulfill this criterion as each flip occurs separately.

The random variable counting the number of successes across these trials follows a binomial distribution. Understanding this distribution is crucial for calculating expected values, variances, and probabilities in scenarios involving multiple identical and independent trials.

Probability of Success

The probability of success, often denoted as \(p\), is a critical concept in probability theory and is central to the binomial distribution. It quantifies the likelihood of a favorable outcome occurring in a single trial of an experiment. For example, if the task is to flip a coin, and getting heads is considered a success, then the probability of success is the probability that the coin will land on heads.
In scenarios involving coins, dice, or similar objects, this probability can be determined by:

• **Understanding limitations:** Ensure that the event being considered (getting heads in this instance) is clearly defined.
• **Evaluating symmetry:** With a fair coin, the probability of heads might be \( \frac{1}{2} \), but this can vary, as with our biased coin example where it's \( \frac{1}{3} \).

The probability of success directly influences calculations involving the binomial distribution. Knowing \(p\) helps determine the expected value, which is given by \(E(X) = n \cdot p,\) where \(n\) is the number of trials.

Coin Flip Probability

Coin flip probability refers to the chance of a coin landing on a particular side—heads or tails—when tossed. This probability is foundational for many probability exercises, especially those involving random variables.

• **Fair vs. biased coins:** A fair coin has equal probabilities, \( \frac{1}{2} \) for both heads and tails. A biased coin, however, will have differing probabilities, such as our example where the chance of heads is \( \frac{1}{3} \).
• **Real-life applications:** Modeling scenarios with coin flips helps in understanding how theoretical probability translates to real-world outcomes. This is useful in situations where outcomes are binary, like success/failure or accept/reject.
• **Experimentation:** Flipping a large number of times and observing the results can provide a practical understanding of these probabilities and their precision.

Coin flip probabilities serve as a stepping stone to comprehend more complex probabilistic models, like the binomial distribution, helping to quantify and predict outcomes effectively.