Question

Given a random variable \(X\), how does the variance of \(c X\) relate to that of \(X\) ?

Short answer

The variance of \(cX\) is \(c^2\) times the variance of \(X\).

Step-by-step solution

Understanding Variance of a Scaled Random Variable

To determine the relationship between the variances of a scaled random variable, you need to know a basic property of variance. This property states that if a random variable \(X\) has a variance \(\text{Var}(X)\), then multiplying \(X\) by a constant \(c\) results in \(cX\) having a variance of \(c^2 \times \text{Var}(X)\).

Applying the Variance Property

Given the above property, we can directly apply it. If \(X\) is a random variable with variance \(\text{Var}(X)\), then the variance of \(cX\) is given by \(\text{Var}(cX) = c^2 \times \text{Var}(X)\).

Key concepts

Random Variables

Random variables are fundamental concepts in probability and statistics that represent numerical outcomes of a random phenomenon. When dealing with random variables, we typically encounter two types: discrete and continuous.
Discrete random variables may take on a finite or countable sequence of values, like rolling a die where outcomes can be 1 through 6. In contrast, continuous random variables can take on any numerical value within a given range, like measuring the exact weight of an apple.
Random variables are crucial because they allow us to define and analyze probability distributions, which describe how probabilities are spread over the possible outcomes.

Variance Property

Variance is a measure of how much a random variable's outcomes differ from its expected value, known as the mean. The formula for variance is given by equation:\[ \text{Var}(X) = E[(X - \mu)^2] \]where \( \mu \) is the expected value of the random variable \( X \), and \( E \) denotes the expectation.
An essential property of variance is how it responds to transformations of the random variable. If a random variable \( X \) has a variance \( \text{Var}(X) \), then altering \( X \) by multiplying it with a constant \( c \), transforms the variance to \( c^2 \times \text{Var}(X) \).
This property is valuable in statistical analysis, enabling easier calculations when dealing with scaled variables, like financial returns or data units.

Scaling Factor in Variance

The scaling factor's effect on variance highlights the profound impact a constant multiplier has when applied to a random variable. Consider a random variable \( X \) with variance \( \text{Var}(X) \). If we scale \( X \) by a constant \( c \), forming a new random variable \( cX \), the variance of \( cX \) adjusts by the square of the scaling factor, expressed as \( c^2 \times \text{Var}(X) \).

• This means that scaling amplifies the variability by the square of \( c \).
• This scaling principle helps in understanding variability in different measurement units or contexts, such as when converting currencies or test scores.

Understanding this concept assists learners in predicting how changes in magnitude affect the spread of a distribution.