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Show that if \(P(A \mid C)>P(B \mid C)\) and \(P\left(A \mid C^{c}\right)>P\left(B \mid C^{c}\right),\) then \(P(A)>P(B)\). Is the converse true? Prove or give a counterexample.

Short Answer

Expert verified
Using the law of total probability, we deduce that if \(P(A \mid C) > P(B \mid C)\) and \(P(A \mid C^c) > P(B \mid C^c)\), then it is true that \(P(A) > P(B)\). The converse is not necessarily true, as a counterexample can be constructed.

Step by step solution

01

Understanding the Given Inequalities

Begin by noting the two given conditional probability inequalities: the first one states that the probability of event A given event C is higher than the probability of event B given event C, written as \(P(A \mid C) > P(B \mid C)\). The second one states that the probability of event A given the complement of event C is higher than the probability of event B given the complement of event C, represented as \(P(A \mid C^c) > P(B \mid C^c)\).
02

Express Probabilities using Total Probability Theorem

Apply the law of total probability to express \(P(A)\) and \(P(B)\) as a sum of probabilities conditioned on C and its complement. The law states that \(P(A)=P(A \mid C)P(C) + P(A \mid C^c)P(C^c)\) and \(P(B)=P(B \mid C)P(C) + P(B \mid C^c)P(C^c)\). These equations represent the total probability of A and B happening, factoring in whether C occurs or not.
03

Substitute and Compare the Equations

Substitute the inequalities into the equations for total probability. Knowing that \(P(A \mid C)P(C) > P(B \mid C)P(C)\) and \(P(A \mid C^c)P(C^c) > P(B \mid C^c)P(C^c)\) due to the given conditions, add these two inequalities to find that \(P(A > P(B)\). This is because the sum of greater quantities is also greater than the sum of their counterparts.
04

Evaluate the Converse

The converse suggests that if \(P(A) > P(B)\), then both \(P(A \mid C)>P(B \mid C)\) and \(P(A \mid C^{c})>P(B \mid C^{c})\) should always be true. To evaluate this, consider possible cases. The converse does not hold absolutely, as one can construct a counterexample where \(P(A) > P(B)\) but one of the conditional probabilities might not follow, say, \(P(A \mid C) < P(B \mid C)\) or \(P(A \mid C^{c}) < P(B \mid C^{c})\).
05

Provide a Counterexample for the Converse

To illustrate a counterexample, let's say events A and B are independent of event C and let \(P(A) = 0.6\), \(P(B) = 0.4\), \(P(C) = 0.5\). It can be seen that \(P(A) > P(B)\), but since A and B are independent of C, \(P(A \mid C) = P(A) > P(B) = P(B \mid C)\), and \(P(A \mid C^c) = P(A) > P(B) = P(B \mid C^c)\). In this case, the condition holds, showing that the converse is not necessarily false. We must find a counterexample where \(P(A \mid C) < P(B \mid C)\) or \(P(A \mid C^{c}) < P(B \mid C^{c})\) even though \(P(A) > P(B)\). Assume A and B are mutually exclusive events, \(P(A) = 0.3, P(B) = 0.2, P(C) = 0.5\), then \(P(A \mid C) = 0.6, P(B \mid C) = 0.4, P(A \mid C^{c}) = 0, P(B \mid C^{c}) = 0\). Here \(P(A) > P(B)\), but \(P(A \mid C) > P(B \mid C)\) is not true for all subsets of the sample space, specifically for \(C^{c}\) contradicting the converse statement.

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Key Concepts

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

Total Probability Theorem
The Total Probability Theorem is a fundamental rule that relates marginal probabilities to conditional probabilities. When dealing with complex problems involving multiple events, it simplifies the process of finding the probability of an event A by considering all possible ways that event A can occur. The theorem can be mathematically expressed as:
\[P(A) = P(A \mid B)P(B) + P(A \mid B^c)P(B^c)\]
where \(B^c\) denotes the complement of event \(B\). This expression means that the total probability of event A happening is the sum of the probability of A occurring given that B occurs, weighted by the probability of B happening, and the probability of A occurring given that B does not occur, weighted by the probability of B not happening. In the exercise provided, the total probability theorem helps demonstrate how the probability of event A exceeds that of event B by considering these events in the contexts of event C and its complement, \(C^c\).
Probability Inequalities
Probability inequalities are a set of rules that help us to understand the relationship between different probability events and make comparisons. In the case presented, the inequalities \(P(A \mid C) > P(B \mid C)\) and \(P(A \mid C^c) > P(B \mid C^c)\) indicate that whether event C occurs or not, event A is always more likely to occur than event B. These inequalities can be analyzed further using the total probability theorem. When the probabilities are weighted by the probability of event C and its complement, we can sum these inequalities to conclude that \(P(A) > P(B)\). However, it is important to note that while these inequalities help in deducing the relationship between the marginal probabilities of A and B, the converse relationship may not always hold true. This hints at the inherent limitations that probability inequalities possess and the need to examine each claim case by case.
Counterexample
A counterexample is a specific case which shows that a general statement is not always true. It plays a crucial role in mathematical proofs, serving both to disprove conjectures and to illustrate the boundaries within which a given statement remains valid. In our exercise, while it's shown that given the conditions, \(P(A) > P(B)\) is true, the converse is not necessarily true. To disprove the converse, one would need to find an instance where despite \(P(A) > P(B)\), the conditional probabilities do not follow the expected inequality. A carefully constructed example illustrates this point—considering mutually exclusive events A and B, and an independent event C, it is possible to find probabilities for these events such that \(P(A) > P(B)\) but one or more of the conditional inequalities are not satisfied. This demonstrates the necessity to assess each situation individually and the importance of counterexamples in understanding the scope of mathematical propositions.

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Most popular questions from this chapter

Polya's urn scheme for a contagious disease. An urn contains initially \(b\) black balls and \(r\) red balls \((r+b=n) .\) A ball is drawn on an equally likely basis from among those in the urn, then replaced along with \(c\) additional balls of the same color. The process is repeated. There are \(n\) balls on the first choice, \(n+c\) balls on the second choice, etc. Let \(B_{k}\) be the event of a black ball on the \(k\) th draw and \(R_{k}\) be the event of a red ball on the \(k\) th draw. Determine a. \(P\left(B_{2} \mid R_{1}\right)\) b. \(P\left(B_{1} B_{2}\right)\) c. \(P\left(R_{2}\right)\) d. \(P\left(B_{1} \mid R_{2}\right)\).

Since \(P(\cdot \mid B)\) is a probability measure for a given \(B,\) we must have \(P(A \mid B)+P\left(A^{c} \mid B\right)=1\). Construct an example to show that in general \(P(A \mid B)+P\left(A \mid B^{c}\right) \neq 1\).

In a certain population, the probability a woman lives to at least seventy years is 0.70 and is 0.55 that she will live to at least eighty years. If a woman is seventy years old, what is the conditional probability she will survive to eighty years? Note that if \(A \subset B\) then \(P(A B)=P(A)\).

Data on incomes and salary ranges for a certain population are analyzed as follows. \(S_{1}=\) event annual income is less than $$\$ 25,000 ; S_{2}=$$ event annual income is between $$\$ 25,000$$ and $$\$ 100,000 ; S_{3}=$$ event annual income is greater than $$\$ 100,000 . E_{1}=$$ event did not complete college education; \(E_{2}=\) event of completion of bachelor's degree; \(E_{3}=\) event of completion of graduate or professional degree program. Data may be tabulated as follows: \(P\left(E_{1}\right)=0.65, P\left(E_{2}\right)=0.30,\) and \(P\left(E_{3}\right)=0.05\). \(P\left(S_{i} \mid E_{j}\right)\) $$\begin{array}{|l|l|l|l|}\hline & S_{1} & S_{2} & S_{3} \\ \hline E_{1} & 0.85 & 0.10 & 0.05 \\\\\hline E_{2} & 0.10 & 0.80 & 0.10 \\ \hline E_{3} & 0.05 & 0.50 & 0.45 \\\\\hline P\left(S_{i}\right) & 0.50 & 0.40 & 0.10 \\\\\hline\end{array}$$ a. Determine \(P\left(E_{3} S_{3}\right)\). b. Suppose a person has a university education (no graduate study). What is the (conditional) probability that he or she will make $$\$ 25,000$$ or more? c. Find the total probability that a person's income category is at least as high as his or her educational level.

Five percent of the units of a certain type of equipment brought in for service have a common defect. Experience shows that 93 percent of the units with this defect exhibit a certain behavioral characteristic, while only two percent of the units which do not have this defect exhibit that characteristic. A unit is examined and found to have the characteristic symptom. What is the conditional probability that the unit has the defect, given this behavior?

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