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Probability models? In each of the following situations, state whether or not the given assignment of probabilities to individual outcomes is legitimate, that is, satisfies the rules of probability. If not, give specific reasons for your answer. (a) Roll a die and record the count of spots on the up-face: \(P(1)=0, P(2)=1 / 6, P(3)=1 / 3, P(4)=1 / 3,\) \(P(5)=1 / 6, P(6)=0\) (b) Choose a college student at random and record gender and enrollment status: \(P(\text { female full-time })=\) \(0.56, P(\text { male full -time })=0.44, P(\text { female part-time })=\) \(0.24, P(\text { male part-time })=0.17\) (c) Deal a card from a shuffled deck: \(P(\text { clubs })=\) \(12 / 52, P\) (diamonds \()=12 / 52, P(\text { hearts })=12 / 52\) \(P(\text { spades })=16 / 52\) .

Short Answer

Expert verified
(a) Legitimate; (b) Not legitimate; (c) Legitimate.

Step by step solution

01

Analyze the Probability Rules

The probability of any event is a number between 0 and 1, inclusive. The sum of the probabilities of all possible outcomes must be equal to 1.
02

Evaluate Case (a) - Rolling a Die

In rolling a die, there are 6 possible outcomes: 1, 2, 3, 4, 5, 6. The probability assigned to each outcome must be between 0 and 1.Check the sum of given probabilities: \[P(1) + P(2) + P(3) + P(4) + P(5) + P(6) = 0 + \frac{1}{6} + \frac{1}{3} + \frac{1}{3} + \frac{1}{6} + 0\]Calculate this sum:\[\frac{1}{6} + \frac{1}{3} + \frac{1}{3} + \frac{1}{6} = \frac{1}{6} + \frac{2}{6} + \frac{2}{6} + \frac{1}{6} = 1\]Since the sum is 1, the assignment is legitimate.
03

Evaluate Case (b) - Choosing a College Student

Probabilities must sum to 1 for all possible outcomes. Check the sum:\[ P(\text{female full-time}) + P(\text{male full-time}) + P(\text{female part-time}) + P(\text{male part-time}) = 0.56 + 0.44 + 0.24 + 0.17\]Calculate this sum:\[0.56 + 0.44 + 0.24 + 0.17 = 1.41\]Since the total probability exceeds 1, this is not a legitimate probability model.
04

Evaluate Case (c) - Dealing a Card from the Deck

The deck contains 52 cards, and probabilities must add up to 1.Calculate the sum:\[P(\text{clubs}) + P(\text{diamonds}) + P(\text{hearts}) + P(\text{spades}) = \frac{12}{52} + \frac{12}{52} + \frac{12}{52} + \frac{16}{52}\]Calculate this sum:\[\frac{12}{52} + \frac{12}{52} + \frac{12}{52} + \frac{16}{52} = \frac{52}{52} = 1\]Since the sum equals 1, the assignment is legitimate.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Probability Rules
Probability rules are essential to understanding and determining whether a given set of probabilities is valid. These rules help you assess if an assignment of probabilities is legitimate. The foundation of probability rules is built on a few key principles.

First, the probability of any event must be a number between 0 and 1. This range reflects the fact that the event cannot occur less than "never" (0) or more than "always" (1). Any assignment of probabilities that falls outside of this range is automatically considered invalid.
Next, the sum of probabilities for all possible outcomes must equal exactly 1. This is because the total probability must represent certainty that one of the possible outcomes will occur. If the sum is less than 1, that suggests some possible outcomes have been omitted. If it's greater than 1, it indicates there's an over-accounting of possibilities.

Understanding these rules is primary to evaluating probability distributions and ensures accurate mathematical reasoning.
Legitimate Probability Distribution
A legitimate probability distribution is characterized by certain criteria that must be met. It's crucial in making accurate predictions and conclusions based on probability.

The first criterion is that each individual probability in the distribution should lie between 0 and 1, inclusive. This requirement ensures that probabilities are sensibly reflecting the likelihood of an event's occurrence.
Another critical aspect of a legitimate distribution is that the sum of all probabilities associated with potential outcomes must equal 1. This guarantees that the distribution accounts for all possibilities, neither omitting any events nor overestimating the likelihood of outcomes.

To determine if a distribution is legitimate, you systematically check that each probability meets these criteria, and verify that their total sums to 1. Successfully doing this ensures adherence to fundamental probability rules.
Sum of Probabilities
The sum of probabilities is a vital concept when evaluating probability models. It determines if an entire set of probabilities accounts for all potential outcomes and respects the boundaries of legitimate probability distributions.

When we calculate the sum of probabilities, we are checking if all the possible outcomes, when added together, equal 1. For example, in rolling a die, the probabilities of 1, 2, 3, 4, 5, and 6 must add up to 1. If they do, the distribution is valid and encompasses all possible scenarios.
If the sum exceeds 1, it indicates an overlap or error in assigning probabilities, signifying that some events have been double-counted or exaggerated. If less than 1, it suggests missing possible outcomes, possibly making the model incomplete.

One of the quickest checks for any probability distribution's legitimacy is comparing this sum to 1, providing a clear pathway to assessing its correctness.
Outcomes Evaluation
Outcomes evaluation is the process of assessing the probability assignment in a model to ensure that it accurately represents all possible outcomes.

Consider rolling a die. To evaluate the outcomes, you check each probability assigned to the numbers on the die (1 through 6). Ensuring every individual probability is between 0 and 1 is essential, but, more importantly, evaluating that they all sum to 1 will determine the legitimacy.
Evaluating outcomes involves assessing both quantitative and qualitative components of the distribution. Quantitatively, calculating the sum ensures all outcomes are accounted for. Qualitively, understanding the context—like rolling a die or drawing cards—helps verify all potential scenarios have been correctly identified.

Every evaluation of outcomes helps confirm if the probability model is valid, offering certainty about one's assumptions in statistical practices.

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Most popular questions from this chapter

Tall people and basketball players Select an adult at random. Define events T: person is over 6 feet tall, and B: person is a professional basketball player. Rank the following probabilities from smallest to largest. Justify your answer. $$ P(T) \quad P(B) \qquad P(T | B) \qquad P(B | T) $$

Role-playing games Computer games in which the players take the roles of characters are very popular. They go back to earlier tabletop games such as Dungeons & Dragons. These games use many different types of dice. A four-sided die has faces with 1, 2, 3, and 4 spots. (a) List the sample space for rolling the die twice (spots showing on first and second rolls). (b) What is the assignment of probabilities to out- comes in this sample space? Assume that the die is perfectly balanced.

Is this valid? Determine whether each of the following simulation designs is valid. Justify your answer. (a) According to a recent poll, 75% of American adults regularly recycle. To simulate choosing a random sample of 100 U.S. adults and seeing how many of them recycle, roll a 4-sided die 100 times. A result of 1, 2, or 3 means the person recycles; a 4 means that the person doesn’t recycle. (b) An archer hits the center of the target with 60% of her shots. To simulate having her shoot 10 times, use a coin. Flip the coin once for each of the 10 shots. If it lands heads, then she hits the center of the target. If the coin lands tails, she doesn’t.

The birthday problem What’s the probability that in a randomly selected group of 30 unrelated people, at least two have the same birthday? Let’s make two assumptions to simplify the problem. First, well ignore the possibility of a February 29 birthday. Second, we assume that a randomly chosen person is equally likely to be born on each of the remaining 365 days of the year. (a) How would you use random digits to imitate one repetition of the process? What variable would you measure? (b) Use technology to perform 5 repetitions. Record the outcome of each repetition. (c) Would you be surprised to learn that the theoretical probability is 0.71? Why or why not?

In an effort to find the source of an outbreak of food poisoning at a conference, a team of medical detectives carried out a study. They examined all 50 people who had food poisoning and a random sample of 200 people attending the conference who didn’t get food poisoning. The detectives found that 40% of the people with food poisoning went to a cocktail party on the second night of the conference, while only 10% of the people in the random sample attended the same party. Which of the following statements is appropriate for describing the 40% of people who went to the party? (Let F = got food poisoning and A = attended party.) $$ \begin{array}{ll}{\text { (a) } P(F | A)=0.40} & {\text { (d) } P\left(A^{C} | F\right)=0.40} \\ {\text { (b) } P\left(A | F^{C}\right)=0.40} & {\text { (e) None of these }} \\ {\text { (c) } P\left(F | A^{C}\right)=0.40}\end{array} $$

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