Strength of Endpoints Measured
The scientific strength of a study's findings is determined by the endpoint(s) measured. In the classification system, a progressive alphabetic scale is used to indicate the scientific strength of endpoints, with the letter A assigned to the strongest endpoint that can be measured and the letter D assigned to the weakest endpoint. Commonly measured endpoints in human cancer treatment studies are listed below in descending order of strength:
- Total mortality: The proportion of the study population that died. Frequently called the death rate. Measured from a defined point in time, such as the time of diagnosis or the time since treatment was initiated. This is the most easily defined and objective endpoint. The inverse of total mortality, i.e., overall survival, may be the reported value.
- Cause-specific mortality: Death from a specified cause in the population under study, for example, death from cancer versus death from side effects of therapy versus death from other causes. This endpoint is more subjective than total mortality. When death from disease (e.g., cancer, heart disease, etc.) is the measured endpoint, the inverse value, i.e., disease-specific survival, may be reported instead.
- Carefully assessed quality of life: Although a very subjective endpoint, quality of life is an extremely important endpoint to patients. The strength of a quality of life assessment depends on the validity of the instruments (i.e., questionnaires, psychologic tests, etc.) used.
- Indirect surrogates: These are measures that substitute for actual health outcomes, and they are subject to investigator interpretation. In descending order of strength, indirect surrogates include the following:
- Disease-free survival: Length of time no cancer was detected after treatment.
- Progression-free survival: Length of time disease was stable or did not get worse after treatment.
- Tumor response rate: The proportion of patients whose tumors responded to treatment and the degree or extent to which the tumors responded.