Attempts to quantitate clinical conditions on a graduated scale, thereby facilitating comparison among populations examined by the same criteria and methods is:
"A numerical value describing the relative status of a population on a graduated scale with definite upper and lower limits, which is designed to permit and facilitate comparison with other populations classified by the same criteria and methods". - Russell A.L. 
"Epidemiologic indices are attempts to quantitate clinical conditions on a graduated scale, thereby facilitating comparison among populations examined by the same criteria and methods" - Irving Glickman.
Indices
Survey
Biostatistics
Analysis