Question

An appliance dealer sells three different models of upright freezers having \(13.5,15.9\), and \(19.1\) cubic feet of storage space. Let \(x=\) the amount of storage space purchased by the next customer to buy a freezer. Suppose that \(x\) has the following probability distribution: $$ \begin{array}{lrrr} x & 13.5 & 15.9 & 19.1 \\ p(x) & .2 & .5 & .3 \end{array} $$ a. Calculate the mean and standard deviation of \(x\). b. If the price of the freezer depends on the size of the storage space, \(x\), such that Price \(=25 x-8.5\), what is the mean value of the variable Price paid by the next customer? c. What is the standard deviation of the price paid?

Short answer

The mean storage size purchased \(E(x) = sum(x*p(x))\) and the standard deviation \(SD(x) = sqrt(E(x^2) - {E(x)}^2)\). The expected price is given by \(E(Price) = 25*E(x) - 8.5\) and the standard deviation of the price \(SD(Price) = 25*SD(x)\).

Step-by-step solution

Calculate the mean of x

To find the mean of x, multiply each possible value by its probability, then add all these products up. The mean (expected value) \(E(x)\) can then be calculated like this: \[E(x) = \sum x*p(x) = 13.5*.2 + 15.9*.5 + 19.1*.3\].

Calculate the standard deviation of x

The standard deviation is the square root of the variance, and the variance in terms of probability is: \[Var(x) = E(x^2) - {E(x)}^2\]. So first we compute \(E(x^2)\) which is \[E(x^2) = \sum x^2*p(x) = 13.5^2*.2 + 15.9^2*.5 + 19.1^2*.3\] Then the variance is calculated and afterwards the standard deviation, which is the square root of the variance.

Calculate the mean of Price

Here, Price is a function of x. So we can use the linearity of the expected value, which gives us \(E(Price) = E(25x - 8.5) = 25*E(x) - 8.5\). We already calculated \(E(x)\) in Step 1.

Calculate the standard deviation of Price

For functions of a random variable, the standard deviation does not maintain linearity. Thus, to find the standard deviation of Price, we calculate the variance first. Since Price = 25*x - 8.5, the variance of Price is \(Var(Price) = {25}^2 * Var(x)\), where Var(x) is the variance of x calculated in Step 2. The standard deviation then is the square root of the variance.

Key concepts

Mean Calculation

The mean of a probability distribution is a critical concept in statistics, representing the average outcome if an experiment is repeated many times. To calculate the mean, also referred to as the expected value of a random variable, each possible value the variable can assume is weighted by the probability of that value occurring. In practical terms, if a customer is choosing between freezers with different storage spaces, and each space has an associated probability of being chosen, the mean storage space will give us an idea of the 'average' choice a customer makes.

For example, given the values of storage space, \(13.5, 15.9, 19.1\) cubic feet, and their probabilities, \(0.2, 0.5, 0.3\), the mean is calculated by summing up the product of each storage space and its probability: \[\text{Mean} = 13.5 \times 0.2 + 15.9 \times 0.5 + 19.1 \times 0.3\]. This weighted sum gives us the expected value which represents the average storage space a customer would buy.

Standard Deviation Calculation

While the mean tells us the average, the standard deviation is a measure of how spread out the numbers are in a probability distribution. It indicates the typical distance of the values from the mean. To compute the standard deviation, we begin with the variance, which represents the mean of the squared differences from the expected value. To clarify, if our random variable represents freezer storage space, a low variance would mean customers tend to choose sizes close to the mean size, while a high variance indicates a wide variety of sizes being chosen.

The formula estimates the variance by summing up the squares of each value times its probability and then subtracting the square of the mean: \[\text{Variance} = E(x^2) - {E(x)}^2\]. After determining the variance, we take its square root to find the standard deviation, giving us a clear indication of the variability in customers' choices of freezer size.

Expected Value of Random Variable

The expected value of a random variable is similar to the mean, but it's used more explicitly in the context of probability distributions. It represents the theoretical long-term average of the outcomes after many trials of a random process. For instance, when determining which size of freezer a customer will buy, the expected value gives us an idea of what to anticipate on average over many sales.

To calculate it, we follow a simple process: multiply each possible value of the random variable by its probability, then add all these products together. The expected value is often denoted by \(E(x)\) and not only does it provide an average, but it's also foundational in other calculations within probability theory, such as finding the variance or establishing expected returns in finance.

Variance of a Random Variable

The variance is a numerical value that represents how measurements in a set are spread out from their expected value, or mean. In the context of probability distributions, the variance quantifies the dispersion of possible outcomes. A small variance indicates that the numbers are not far from the mean, the outcomes are consistent, and thus less risky. Conversely, a large variance signifies a wide range of outcomes and higher risk.

To compute the variance, as we did with the freezer sizes, we determine the expected value of the squares of the differences between each outcome and the mean. Formally, this is written as \[\text{Variance} = E(x^2) - {E(x)}^2\]. The variance provides us with a quantitative measure of uncertainty in any process involving random variables, serving as a foundational concept for more advanced statistical measures like the standard deviation.