••• CHAPTER 64^ Basic Epidemiology and Clinical Biostatistics^641
Risk is the likelihood or probability that a person will contract a disease. A simple formula for calculating this is as
follows:
R = A/N where R is risk, A is the number of newly affected persons, and N is the number of unaffected persons
under observation.❍ How is the number of newly diagnosed cases per year for a disease determined?
Numbers of newly diagnosed cases are affected by (1) the frequency with which the disease occurs, (2) how the
disease is defined, (3) the size of the population from which cases develop, and (4) completeness of case reporting.
❍ Why is disease screening performed?
It is done in order to detect a disease at an earlier stage than would occur through routine methods. Screening can
detect people who either have a disease that can be treated in the early stages or those individuals at high risk of the
disease who would benefit from early intervention to reduce risk of developing the disease.
❍ Why is it important for a screening test to be reliable and valid?
A reliable test gives the same results when the test is repeated on the same person several times; results are
reproducible. A valid test measures what it is designed to measure. A reliable and valid screening test minimizes the
number of false-positive and false-negative test results.
❍ How is the validity of a screening test assessed?
Sensitivity and specificity are used to assess the validity of a screening test. Sensitivity is the proportion of people
with a disease or condition with a positive test result. Specificity is the proportion of people without a disease or
condition with a negative test result. The goal is to maximize both sensitivity and specificity to minimize false
positives and false negatives. Sensitivity and specificity are also used to assess diagnostic tests.
❍ What is the predictive value and how does it relate to a screening test?
The predictive value of a test is the ability of the test to accurately distinguish people with and without a disease
or condition. The positive predictive value is the probability of disease given a positive test result. The negative
predictive value is the probability of no disease or condition given a negative test result. Positive and negative
predictive values are also used to assess diagnostic tests.
❍ What is a likelihood ratio?
Unlike predictive values, likelihood ratios are not influenced by the prevalence of the disease in the population
being studied. A positive likelihood ratio is how likely it is to obtain a given test result among people with the
disease. A negative likelihood ratio is how likely it is to obtain a given test result among people without the disease.
❍ What is a likelihood ratio nomogram?
A likelihood ratio nomogram is a graphical scale used to determine the posttest probability of disease. It utilizes the
likelihood ratio of a diagnostic test and the pretest probability of a disease (disease prevalence in a population) to
determine the posttest probability of a patient having the disease.
❍ What is an ROC curve used for?
A receiver operating characteristic (ROC) curve is used to evaluate the properties of a diagnostic test. It is a plot of
the true-positives (sensitivity) on the y-axis and the false-positives (1-specificity) on the x-axis. The area under the
ROC curve (AUC) is calculated and compared to 1. Values closest to 1 indicate a good diagnostic test.