Sensitivity:  If a person has a disease, how often will the test be positive (true positive rate)?  Put another way, if the test is highly sensitive and the test result is negative you can be nearly certain that they dont have disease.  A Sensitive test helps rule out disease (when the result is negative). Sensitivity rule out or "Snout" Sensitivity= true positives/(true positive + false negative) Specificity:  If a person does not have the disease how often will the test be negative (true negative rate)? In other terms, if the test result for a highly specific test is positive you can be nearly certain that they actually have the disease. A very specific test rules in disease with a high degree of confidence Specificity rule in or "Spin". Specificity=true negatives/(true negative + false positives) Predictive value for a positive result (PV+): PV+ asks " If the test result is positive what is the probability that the patient actually has the disease?" PV+= true positive/(true positive + false positive) Predictive value for a negative result (PV-): PV- aks " If f the test result is negative what is the probability that the patient does not have disease?" PV-= true negatives/(true negatives +false negatives)