Question

of 200 positives results 175 are from patients with the disease under investigation. of 1000 negative results 100 are from patients with disease. Assume the other 25 positives are from individuals without the disease and the other 900 negative results are also from patients without the disease.

For the sensitivity without changing prevalence to 5% I got 63.6. For the specificity I got 97.3. ....I am not sure If I am correct

1.Change the prevalence to 5%

a. what is the sensitivity?

b.what is the specificity?

I am having a hard time finding specificity and sensitivity when the prevalence is changed to 5%

Answer 1

For the given data, sensitivity = Number of true positive results / Number of people with disease

= 175/(175+100)

= 0.636

= 63.6%

Specificity = Number of true negative results / Number of people without disease

= 900/(900+25)

= 0.973

= 97.3%

1. When prevalence is 5%,

a) Sensitivity = 63.6%

b) Specificity = 93.7%

(Sensitivity and specificity are independent of prevalence. These two are properties of a test and will remain constant for all values of prevalence)