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Nathaniel E. Knox Cartwright, MRCOphth

	Since this article does not have an abstract, we have provided the first 150 words of the full text and any section headings.

In their recent letters, both Mathers et al¹ and Schein and Katz² incorrectly suggested that the 30-year incidence of ulcerative keratitis can be determined by multiplying the annual incidence of this condition by 30 if the annual incidence is assumed to be constant.

If the long-term incidence or cumulative probability (P) were equal to annual incidence (i) multiplied by time in years (t), P would rise linearly over time and eventually exceed certainty (100%, or 1.0). Such a situation is clearly impossible. In reality, P increases in a logarithmic or inverse exponential fashion tending toward but never reaching 100%.

To calculate P, the probability of not being affected is subtracted from certainty. In any one year, the probability of not being affected is 1 – i. Over t years, the same probability is (1 – i)^t . Therefore, P. . . [Full Text of this Article]

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RELATED LETTERS

Accurate Calculation of Longer-term Incidences From Short-term Incidence Values—Reply
Hall T. McGee and William D. Mathers
Arch Ophthalmol. 2008;126(4):580.
EXTRACT | FULL TEXT  

Accurate Calculation of Longer-term Incidences From Short-term Incidence Values—Reply
Oliver D. Schein and Joanne Katz
Arch Ophthalmol. 2008;126(4):580.
EXTRACT | FULL TEXT