It is often said that the structural damage due to glaucoma precedes functional loss, it is not always clear in its interpretation. To define, which test shows up the earliest sign of glaucomatous damage would be difficult to predict, since the outcome can be influenced by many factors, such as the sensitivity of the test, inter - individual difference or the stage of the disease process itself. Studies on post-mortem retinal ganglion cells counts and SAP field loss in humans and monkeys lead to an observation that, on an average about 25% retinal ganglion cells loss leads to development of an afferent pupillary defect, 35% loss for visual field defects and 40% loss leads to worsening of visual acuity. It would be safe to state that a) the most important clinical challenge is early detection of glaucomatous damage and progression over a period of time; b) Both structural and functional tests are important in assessing early damage and progression; c) significant damage to the retinal ganglion cells can occur before standard tests detect a functional loss in vision. These have paved the way to the development of many studies on the structure - function correlation and stating linear models for the same. However these studies present a dilemma; is the structure - function relation on glaucoma, a statistical statement (a structural measure will reach significance before a functional test) or relational statement (statistical correlation between the structural and functional tests).