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What is In a town of population 5000, 500 are already myopic on January 1, 2011. The number of new myopia cases is 90 till it December 31, 2011. Calculate the incidence of Myopia in the town in 2011- and explain in detail?
Out of 5000, 500 are already myopic. Thus, population at risk in the question = 5000 - 500 = 4500

Incidence = New cases in the year / Population at risk in the year = 90 / 4500 = 0.02