We investigate the degree to which a reduction in ocular sunlight ultra-violet B (UVB) exposure mediates a relationship between wearing eyeglasses and a decreased risk of cataracts. An estimand is proposed in which causal effects are estimated locally within strata based on potential UVB exposure without glasses and the degree to which glasses use reduces UVB exposure. We take advantage of the structure of the data in which the counterfactual UVB exposures if the participants in the study who wore glasses had not worn glasses are considered observable.
Egleston, Brian; Scharfstein, Daniel O.; Munoz, Beatriz; and West, Sheila, "INVESTIGATING MEDIATION WHEN COUNTERFACTUALS ARE NOT METAPHYSICAL: DOES SUNLIGHT UVB EXPOSURE MEDIATE THE EFFECT OF EYEGLASSES ON CATARACTS?" (July 2006). Johns Hopkins University, Dept. of Biostatistics Working Papers. Working Paper 113.