This paper proposes a new index for rating diagnostic tests. Diagnostic tests aim to correctly identify diseased individuals. They may make errors, such as false negatives (negative results for diseased individuals) and false positives (positive results for healthy individuals). The proposed index is given by the formula (ad - bc)/[(a + b)(c + d)], where 'a' is the number of correctly diagnosed diseased individuals, 'b' is the number of false negatives, 'c' is the number of false positives, and 'd' is the number of correctly diagnosed healthy individuals. The index ranges from 0 to 1. A value of 0 indicates the test has the same proportion of positive results for both diseased and healthy individuals, making it ineffective. A value of 1 indicates no errors. The index is independent of the size of the groups and is used to compare the performance of different diagnostic tests. The standard error of the index is calculated to assess the reliability of the index. The paper also discusses the use of confidence intervals and the t-test to compare two diagnostic tests. The index allows for an objective comparison of diagnostic tests and helps determine whether a modification of a test leads to an improvement in its performance. The index is calculated based on the proportion of correctly diagnosed individuals in both diseased and healthy groups. The paper concludes that the index is a useful tool for evaluating the performance of diagnostic tests.This paper proposes a new index for rating diagnostic tests. Diagnostic tests aim to correctly identify diseased individuals. They may make errors, such as false negatives (negative results for diseased individuals) and false positives (positive results for healthy individuals). The proposed index is given by the formula (ad - bc)/[(a + b)(c + d)], where 'a' is the number of correctly diagnosed diseased individuals, 'b' is the number of false negatives, 'c' is the number of false positives, and 'd' is the number of correctly diagnosed healthy individuals. The index ranges from 0 to 1. A value of 0 indicates the test has the same proportion of positive results for both diseased and healthy individuals, making it ineffective. A value of 1 indicates no errors. The index is independent of the size of the groups and is used to compare the performance of different diagnostic tests. The standard error of the index is calculated to assess the reliability of the index. The paper also discusses the use of confidence intervals and the t-test to compare two diagnostic tests. The index allows for an objective comparison of diagnostic tests and helps determine whether a modification of a test leads to an improvement in its performance. The index is calculated based on the proportion of correctly diagnosed individuals in both diseased and healthy groups. The paper concludes that the index is a useful tool for evaluating the performance of diagnostic tests.