Question

Compute the variance \(\operatorname{Var}(X)\) of the random variable \(X\) that counts the number of heads in four flips of a coin that lands heads with a frequency of \(1 / 3 .\)

Short answer

The variance \(\operatorname{Var}(X)\) is \(\frac{8}{9}\).

Step-by-step solution

Understand the Random Variable

The random variable \(X\) represents the number of heads observed in 4 flips of a biased coin, where the probability \(p\) of getting a head is \(\frac{1}{3}\) and the probability \(q = 1 - p = \frac{2}{3}\) of getting a tail.

Recognize the Distribution Type

Since we are flipping a coin 4 times and counting the number of heads, \(X\) follows a binomial distribution with parameters \(n = 4\) (the number of trials) and \(p = \frac{1}{3}\) (the probability of success on each trial).

Recall the Variance Formula for Binomial Distribution

For a binomial distribution \(X \sim \text{Binomial}(n, p)\), the variance \(\operatorname{Var}(X)\) is given by \(npq\), where \(q = 1 - p\).

Calculate the Variance

Using the formula \(\operatorname{Var}(X) = npq\), substitute \(n = 4\), \(p = \frac{1}{3}\), and \(q = \frac{2}{3}\): \[ \operatorname{Var}(X) = 4 \times \frac{1}{3} \times \frac{2}{3} = \frac{8}{9} \]

Key concepts

Variance

Variance is a measure that tells us how much the values of a random variable are spread out. In simpler terms, it provides an idea of how much the outcomes vary from the expected average. When rolling a die or flipping a coin, the variance helps to quantify the unpredictability of these events.

For a binomial distribution, which deals with a fixed number of independent trials of a binary outcome (like flipping a coin), the formula for variance is \(npq\). In this context:

• \(n\) is the number of trials
• \(p\) is the probability of success (such as getting a head)
• \(q = 1 - p\) is the probability of failure (like getting a tail)

For our exercise, where four coin flips are involved and the coin lands heads with a frequency of \(\frac{1}{3}\), the variance is calculated using the formula: \[ \operatorname{Var}(X) = npq = 4 \times \frac{1}{3} \times \frac{2}{3} = \frac{8}{9} \]This result tells us how much the number of observed heads might fluctuate from the average or expected values when the coin is flipped four times.

Random Variable

A random variable is a mathematical construct used to quantify outcomes of a random phenomenon. It's a convenient way to model situations with uncertain results in a manageable mathematical framework.

For example, when flipping a coin, the outcome can be a head or a tail. Here, our random variable \(X\) counts the number of heads in four flips of a biased coin.

Since each flip is an independent event, meaning the outcome of one flip does not affect the others, we define \(X\) to take values within a specific range based on given probabilities:

• 0 heads: All flips result in tails
• 1 head: 1 out of 4 flips result in a head
• 2 heads: 2 out of 4 flips result in heads
• 3 heads: 3 out of 4 flips result in heads
• 4 heads: All flips result in heads

This variable, \(X\), follows a binomial distribution as it involves multiple trials with two possible results (head or tail) in each trial. Modeling problems like this helps predict the likelihood of various outcomes, which is especially useful in fields like statistics and probability.

Probability

Probability measures how likely an event is to occur. It's a way of quantifying uncertainty. In the context of our exercise, probability helps us determine the likelihood of observing a specific number of heads when flipping a biased coin.

The biased coin means that the outcomes (head or tail) do not have an equal chance of occurring. In our example:

• The probability \(p\) of flipping a head is \(\frac{1}{3}\)
• The probability \(q = 1 - p\) of flipping a tail is \(\frac{2}{3}\)

These probabilities allow us to predict the expected outcomes over multiple coin flips. When using binomial distribution in probability:

• Each flip is a trial.
• Each trial has a binary outcome — success (head) or failure (tail).
• We seek to find out the probability of getting 0 to 4 heads in four flips.

Understanding these basic principles enables to better model and predict various outcomes, making the study of probability essential in both theoretical and practical applications.