Question

Compute the variance $\mathrm{Var}(X)$ of the random variable $X$ that counts the number of heads in four flips of a fair coin.

Short answer

The variance $\mathrm{Var}(X)$ is 1.

Step-by-step solution

Define the Random Variable

The random variable $X$ counts the number of heads in four flips of a fair coin. Each flip is a Bernoulli trial with $p=0.5$ (probability of getting heads). Thus, $X$ follows a Binomial distribution with parameters $n=4$ and $p=0.5$.

Recall the Variance Formula for Binomial Distribution

For a binomially distributed random variable $X$, the variance $\mathrm{Var}(X)$ is given by the formula $\mathrm{Var}(X)=n\cdot p\cdot (1-p)$.

Substitute the Values

Substitute $n=4$ and $p=0.5$ into the variance formula: $$\mathrm{Var}(X)=4\cdot 0.5\cdot (1-0.5)$$.

Calculate the Result

Solve the expression $4\cdot 0.5\cdot 0.5$ to find $\mathrm{Var}(X)$. This simplifies to $4\cdot 0.25=1$.

Write Down the Conclusion

After computing the expression, we find that the variance $\mathrm{Var}(X)=1$.

Key concepts

Binomial Distribution

The binomial distribution is a fundamental concept in probability theory. It is used to model the number of successful outcomes in a fixed number of independent experiments, or trials, where each trial has the same probability of success. For example, if we flip a fair coin four times, we might be interested in counting how many times it lands on heads.

In our case, each coin flip represents a trial, and the probability of getting a head (success) is 0.5. The binomial distribution is characterized by two parameters: $n$, the number of trials, and $p$, the probability of success. For coin flips, $n=4$ and $p=0.5$.

• "Success" is getting a head on a coin flip.
• It has a defined number of trials, like our four flips.
• Each trial is independent of others, meaning the outcome doesn't affect others.

The variance in a binomial distribution helps us understand the spread of our results around the expected number of successes. The formula for variance in a binomial distribution is $\mathrm{Var}(X)=n\cdot p\cdot (1-p)$. By plugging our coin parameters into this formula, we determined the variance to be 1. This implies that the number of heads we expect to see is on average evenly distributed around its expected value.

Bernoulli Trial

A Bernoulli trial is one simple kind of experiment where there are only two possible outcomes: success or failure. This is directly applied in our example of flipping a coin.

Each flip of a coin is a Bernoulli trial because it can result in either a head (success) or a tail (failure). In terms of the probability, the success (a head) has a fixed probability $p$, which is 0.5 in this scenario.

• The probability of failure (a tail) is $1-p$, which would also be 0.5 here.
• The assortment of successes and failures in a series of Bernoulli trials forms a binomial distribution.
• These trials are foundational for understanding many types of probability models.

Understanding each flip as a Bernoulli trial helps in calculating probabilities over multiple trials and is an integral concept leading into more complex probability theory topics.

Probability Theory

Probability theory forms the backbone of statistics and is all about quantifying uncertainty. It encompasses a wide range of concepts used to analyze random events and outcomes.

In practical terms, it means understanding that various outcomes of an experiment are not deterministic but random. For the coin flip example, each flip is an event whose result can't be predicted with certainty, but with the help of probability theory, we can analyze and predict the likelihood of different outcomes.

• Probability helps us determine the likelihood of events like getting exactly two heads in four coin flips.
• It underpins statistical inference methods for making predictions and decisions.
• Probability distributions, like the binomial distribution used here, serve as tools for modeling random variables.

Using probability theory, we can translate our real-world observations into mathematical models, allowing us to make assumptions and predictions about a wide range of phenomena. It's essential for fields like finance, medicine, and engineering.