Epidemiology is a fascinating field of study. In fact, large-scale population studies are now relatively common. The conclusions are, however, confusing to the general public at times.
In 2001, the National Eye Institute sponsored AREDS (Age-Related Eye Disease Study) reported that nutritional supplements significantly reduced the progression of early AMD. These supplements included
- 500 milligrams (mg) of vitamin C
- 400 international units (IU) of vitamin E
- 15 mg of beta-carotene
- and 80 mg of zinc oxide
Miller ER 3rd, Pastor-Barriuso R, Dalal D, Riemersma RA, Appel LJ, Guallar E. "Meta-Analysis: High-Dosage Vitamin E Supplementation May Increase All-Cause Mortality." Ann Intern Med. 2004 Nov 10.
Some AMD patients who were on vitamin E supplements went into the panic mode, when in fact the "high-dose" in the 2004 report refers to >>400IU daily intake. The headline writers were somewhat irresponsible.
Ideally, all comparative epidemiology studies employ the same methodology so that the comparisons can be done. One good example is the cataract prevalence studies, most of which use their own home-versions of cataract classification. Your idea of a nuclear cataract-caused blindness may just be my 20/200 vision with a NS3+. So how are we going to compare the data? Eventually, the WHO organized a Cataract Grading Group who published "A Simplified Cataract Grading System" in the September, 2002 issue of Ophthalmic Epidemiology. However, to this day, the WHO system has not been widely adopted, let alone applied. Inertia is probably at the root.
It is really just common sense. For international studies, a common methodology must be agreed upon and strictly adhere to. The results will be far more useful for worldwide public health planning.