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Suppose \(5 \%\) of all people filing the long income tax form seek deductions that they know are illegal, and an additional \(2 \%\) incorrectly list deductions because they are unfamiliar with income tax regulations. Of the \(5 \%\) who are guilty of cheating, \(80 \%\) will deny knowledge of the error if confronted by an investigator. If the filer of the long form is confronted with an unwarranted deduction and he or she denies the knowledge of the error, what is the probability that he or she is guilty?

Short Answer

Expert verified
Answer: The probability that a person is guilty of cheating given that they deny knowledge of the error is approximately \(66.67\%\) or \(\frac{2}{3}\).

Step by step solution

01

Assign events and probabilities

Let's denote the events as follows. 1. \(G\): A person who is guilty of cheating. 2. \(A\): A person who is innocent but made incorrect deductions. 3. \(D\): A person who denies knowledge of the error. Given probabilities are: 1. \(P(G) = 5\% = 0.05\). 2. \(P(A) = 2\% = 0.02\). 3. \(P(D|G) = 80\% = 0.8\).
02

Apply the Bayes' theorem

We need to find the probability \(P(G|D)\) which represents the probability that a person is guilty given that they deny knowledge of the error. According to the Bayes' theorem: \(P(G|D) = \frac{P(D|G) \times P(G)}{P(D)}\)
03

Calculate the probability of denying the error, P(D)

To find \(P(D)\), we need to consider two scenarios: 1. A guilty person denies the error: \(P(D|G) \times P(G)\). 2. An innocent person who made incorrect deductions denies the error: \(P(D|A) \times P(A)\). Assuming innocent people in the population always deny knowledge of the error since they didn't know the error, we get \(P(D|A)=1\). Then: \(P(D) = P(D|G) \times P(G) + P(D|A) \times P(A) = 0.8 \times 0.05 + 1 \times 0.02 = 0.04 + 0.02 = 0.06\)
04

Compute the required probability, P(G|D)

Now, we can substitute the values of \(P(D|G)\), \(P(G)\) and \(P(D)\) in the formula of Bayes' theorem: \(P(G|D) = \frac{P(D|G) \times P(G)}{P(D)} = \frac{0.8 \times 0.05}{0.06} = \frac{0.04}{0.06} = \frac{2}{3}\) So, the probability that a person is guilty of cheating given that they deny knowledge of the error is \(\frac{2}{3}\) or approximately \(66.67\%\).

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Key Concepts

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

Bayes' theorem
Bayes' theorem is a fundamental concept in probability theory that relates the probability of an event, based on prior knowledge of conditions that might be related to the event. To simplify, it allows us to update our beliefs or probabilities as we gain new information.

In the context of the exercise, Bayes' theorem allows us to calculate the probability that a person is guilty of cheating (event G), given that they have denied knowledge of an error (event D). It's essential to note that the theorem is applied here because we have conditional probabilities - probabilities that take into account a specific condition (in this case, the denial of knowledge).

To apply Bayes' theorem, we used the information given in the problem about the respective probabilities of being guilty of cheating, making incorrect deductions, and denying the knowledge of the error. The accuracy of our final probability heavily depends on these given probabilities and our assumption that all innocent people will deny knowledge of the error.

If you're solving a problem that employs Bayes' theorem, you must pay close attention to the events and their associated probabilities. Ensure you have all the needed information and that you place this information correctly into the theorem's formula.
Conditional probability
Conditional probability is the likelihood of an event occurring given that another event has already occurred. This is usually represented as the probability of A given B and written as P(A|B).

In the provided exercise, we utilized conditional probability when we calculated P(D|G), the probability that a guilty person would deny knowledge of the error. Here, the condition is being guilty, and we want to know how that affects the probability of denying the error.

Moreover, conditional probability plays a key role in Bayes' theorem as it involves revising the probability of an event given new evidence. The exercise required us to estimate the probability of guilt after the new evidence (denial) is provided, showing how closely tied these concepts are within probability theory.

Remember that in practice, conditional probabilities can be non-intuitive and require careful interpretation of the problem's context. It's important to identify the condition and the event of interest clearly to accurately calculate the conditional probability.
Probability theory
Probability theory is the branch of mathematics concerned with the analysis of random phenomena. The core objective is to provide a mathematical foundation for understanding and quantifying uncertainty. Probability theory lays the groundwork for various concepts, including event, sample space, probabilities of events, and much more.

The exercise we explored belongs to the realm of probability theory as it deals with the uncertainty of whether individuals filing tax returns are guilty of consciously providing false deductions. By using probability theory concepts such as Bayes' theorem and conditional probability, we can make informed predictions despite the inherent uncertainty.

One of the fundamental lessons from probability theory is that the way you approach a problem can significantly affect your ability to solve it. To improve student comprehension of problems like this, it's crucial to provide a context for the abstract concepts, such as framing the event of interest in a real-world situation (like tax deductions), and emphasizing the relationship between the given data and how we use that to determine probabilities.

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Most popular questions from this chapter

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