system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
In a population of 5000 people, 500 are already myopic on January 1, 2011, number of new myopic cases till December 31, 2011 is 9. Calculate incidence:
assistant
The correct answer is  Incidence = Number of New cases x1000 Total population under risk Incidence = 90/(5000-500)X1000 = 90/4500X1000=2 Ref: Park 22nd ed /p-58.