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. 2016 Sep 19;34(32):3880–3885. doi: 10.1200/JCO.2015.66.2361

Fig A1.

Fig A1.

Misclassification of vasectomy status could have occurred because of under-reporting of vasectomy at enrollment and missed vasectomies after enrollment. The figure illustrates the observed rate ratio as a function of the proportion of men classified as unvasectomized who were actually vasectomized. The form of this function is justified in the following equations. The observed rate ratio comparing vasectomized versus unvasectomized men is: (Rate observed in men classified as vasectomized)/(Rate observed in men classified as unvasectomized). If the true rate of prostate cancer mortality is M in unvasectomized men, and 1.20M in vasectomized men (based on the rate ratio of 1.20 observed in the Health Professional Follow-Up Study), x is the proportion of men classified as unvasectomized who were actually vasectomized, and a weighted average is used to calculate the rate in the denominator in terms of M, then the observed rate ratio is: 1.20/[(1 − x)(1M) + x(1.20M)]. This expression simplifies to 1.20/(1 + 0.20x), which is plotted in the figure. Attenuation of the true rate ratio is relatively modest even when 10% of men classified as unvasectomized were actually vasectomized.