What is the usefulness of detection rate in a confusion matrix? In the R caret documentation for confusionMatrix():  
                        TP
sensitivity    = -----------------
                      TP + FN

                        TP
detection rate = -----------------
                 TP + FP + FN + TN

Practically, why would someone want to calculate detection rate? What is its usefulness?
Many people elsewhere think detection rate is the same as sensitivity. Why is such confusion so common?
I'm a bit confused!
 A: One possible implication for this measure in policy and evaluation is for cost-effectiveness. Suppose, for instance, we intend to implement targeted screening for a disease in a population of healthcare subscribers receiving routine care. The current standard of care is to wait for the patient to present with symptoms of disease in clinic; however, treatment at this point tends to have a low success rate, many complications, and high cost. The rationale for screening is to deliver more timely, less invasive, and more successful treatment at a comparable cost.
If a patient is identified as a true positive, not only must they undergo treatment, but their longer survival requires them to be on treatment for a longer duration, and individuals who would have died from other disease before becoming symptomatic undergo treatment unnecessarily for the disease. The cost of the screening program must incorporate the costs of treating those identified with the disease.
A: It is a parameter that will vary according to the dataset. If you get a high overall performance on a moderate detection rate, it would make me wonder how the performance would vary on a smaller dataset with more positives. 
