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For a job, 555 candidates are drug tested, and their results are tabulated under four categories.

One of these candidates is chosen at random.

Probability gives an idea of how plausible an event is.

It is expressed as

$P\left(E\right)=\frac{\text{Number}\text{of}\text{outcomes}\text{resulting}\text{in}E}{\text{Total}\text{number}\text{of}\text{outcomes}}$

Let A be the event of selecting a person who uses drugs.

Here, the number of people who use drugs is the sum of the two categories:

• The number of people who tested true positive (45)
• The number of people who tested false negative (5)

The total number of persons who got tested is 555.

The probability of event A is

$$\begin{gathered} P\left(A\right)=\frac{Numberofpersonswhousedrugs}{Totalnumberofpersons} \\ =\frac{45+5}{555} \\ =\frac{50}{555} \\ =0.0901 \end{gathered}$$

Therefore, the probability of selecting a person who uses drugs is 0.0901.

The prevalence rate is the number of subjects in the population who are affected by the condition. The actual estimate is the set of subjects who use drugs.

The computed probability does not appear to be a reasonable estimate of the actual prevalence level.