Bifocal adds and environmental temperature.

Eight hundred and eighty-eight optometrists of North and Central America were asked to provide data on their typical bifocal addition (add) for 45-, 50-, 55-, and 60-year-old males and females. Analysis of the 577 responses shows no significant difference of adds with the mean annual temperatures of the optometrists' communities at any of the four age levels. The mean annual temperatures in the survey ranged from -7 degrees Centigrade (19 degrees Fahrenheit) in Alaska to 26 degrees Centigrade (79 degrees Fahrenheit) in Puerto Rico. Previously reported inverse correlations are explained in terms of a self-selection bias by patients at the earlier presbyopic age levels when age of onset criteria are used instead of more advanced presbyopic age levels when all persons seek ophthalmic services. The mean adds for females at all four age levels were between 0.05 and 0.06 D higher than for males, theoretically attributable to differences of physical stature rather than of age of onset or advancement of presbyopia.