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Accurate Calculation of Longer-term Incidences From Short-term Incidence Values—Reply
Hall T. McGee, MD, MS;
William D. Mathers, MD
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In reply
We thank Mr Knox Cartwright for the clarification. As was pointed out, simply multiplying the annual incidence rate by a number of years is not an entirely accurate extrapolation method; however, it is a good approximation when dealing with small numbers.
We used the example of 3.5 cases of bacterial keratitis per 10 000 daily contact lens wearers per year and extrapolated that to 30 years by simply giving the product of the annual incidence and the number of years.1 That gives a total estimated probability of bacterial keratitis in this population of 1.05%. If we had used the formula P = 1 – (1 – i)t as recommended by Mr Knox Cartwright, then the result would have been 1.0447%. This would make a difference of 0.0053% total over 30 years between the 2 methods.
Of course, the point of our original letter . . . [Full Text of this Article]
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