MATH 2311-06 Section 3.1.

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Presentation transcript:

MATH 2311-06 Section 3.1

Random Variables A random variable is a variable whose value is a numerical outcome of a random phenomenon. It assigns one and only one numerical value to each point in the sample space for a random experiment. A discrete random variable is one that can assume a countable number of possible values. A continuous random variable can assume any value in an interval on the number line. A probability distribution table of X consists of all possible values of a discrete random variable with their corresponding probabilities.

Example: Suppose a family has 3 children. Show all possible gender combinations:

Example: Suppose a family has 3 children. Show all possible gender combinations: Keep in mind, there will be (2)(2)(2)=8 combinations. BBB BGB GBB GGB BBG BGG GBG GGG

Example: Suppose a family has 3 children. Now suppose we want the probability distribution for the number of girls in the family. Draw a probability distribution table for this example.

Example: Suppose a family has 3 children. Now suppose we want the probability distribution for the number of girls in the family. Draw a probability distribution table for this example. We need to know the total possible outcomes. We need to categorize them by number of girls. We need a probability of each outcome.

Example: Suppose a family has 3 children. Now suppose we want the probability distribution for the number of girls in the family. Draw a probability distribution table for this example. 0 girls: 1 girl: 2 girls: 3 girls:

Example: Find the following probabilities: 0 girls: 1 girl: 2 girls: 3 girls:

Expected Value: The mean, or expected value, of a random variable X is found with the following formula:

What is the expected number of girls in the family above?

What is the expected number of girls in the family above. In R Studio: What is the expected number of girls in the family above? In R Studio: assign(“x”,c(values)) assign(“p”,c(probabilities)) sum(x*p)

Variance and Standard Deviation Or (the alternate formula) Repeat the Expectancy Formula using x2 instead of x.

Variance and Standard Deviation In R Studio: sum((x-mean)^2*p)

Find the standard deviation for the number of girls in the example above

Example: Poppers…EMCF 3 1. 2. 3. 4. 0.10 0.15 0.30 0.05 0.10 0.15 0.20 0.05 0.50 0.15 0.20 0.25 0.50 0.75 0.80 0.70

Example: 5. What is the expected value? The variance and standard deviation? 3.50 4.20 0.35 5.00 6. 3.66 13.39 1.91 1.55

Rules for means and variances: Suppose X is a random variable and we define W as a new random variable such that W = aX + b, where a and b are real numbers. We can find the mean and variance of W with the following formula:

Rules for means and variances: Likewise, we have a formula for random variables that are combinations of two or more other independent random variables. Let X and Y be independent random variables,

Example: #14 from text: Suppose you have a distribution, X, with mean = 22 and standard deviation = 3. Define a new random variable Y = 3X + 1.