Lyme disease is transmitted by infected ticks. Several tests are available for
people with symptoms of Lyme disease. One of these tests is the EIA/IFA test.
The paper "Lyme Disease Testing by Large Commercial Laboratories in the United
States" (Clinical Infectious Disease [2014]: \(676-681\) ) found that \(11.4 \%\)
of those tested actually had Lyme disease.
Consider the following events:
\(+\) represents a positive result on the blood test
\- represents a negative result on the blood test
\(L\) represents the event that the patient actually has Lyme disease
\(L^{C}\) represents the event that the patient actually does not have Lyme
disease
The following probabilities are based on percentages given in the paper:
$$
\begin{array}{r}
P(L)=0.114 \\
P\left(L^{C}\right)=0.886
\end{array}
$$
$$
\begin{array}{c}
P(+\mid L)=0.933 \\
P(-\mid L)=0.067 \\
P\left(+\mid L^{C}\right)=0.039 \\
P\left(-\mid L^{C}\right)=0.961
\end{array}
$$
a. For each of the given probabilities, write a sentence giving an
interpretation of the probability in the context of this problem.
b. Use the given probabilities to construct a hypothetical 1000 table with
columns corresponding to whether or not a person has Lyme disease and rows
corresponding to whether the blood test is positive or negative.
c. Notice the form of the known conditional probabilities; for example,
\(P(+\mid L)\) is the probability of a positive test given that a person
selected at random from the population actually has Lyme disease. Of more
interest is the probability that a person has Lyme disease, given that the
test result is positive. Use information from the table constructed in Part
(b) to calculate this probability.