Epidemiology module of practical skills for mbbs student

Relationship of Incidence with Prevalence

Prevalence depends upon two factors- the incidence and duration of illness

P = I X D

P= Incidence X mean Duration

Prevalence vs. Incidence

Exercise B
In June, 1995, at the end of their senior year, the following observations were made concerning smoking in a population of 110 graduating seniors. The enrolment for this cohort from 9th to 12th grade was as follows:

9^th Sept. 1991 160

10^th Sept. 1992 140

11^th Sept. 1993 120

12^th Sept. 1994 110
The seniors were asked the following question. “At any time while you were in high school (Sept. ‘91 -June ‘95), did you smoke at least one pack of cigarettes a week? If yes, when did you start and for how long did you smoke?” Of the 110 graduating seniors surveyed, 20 reported having smoked at some time during high school. The following table shows the data for these 20 students:




Using the data above, answer the following questions:

1. 
   From Sept. 1993 - Aug. 31, 1994 what was the incidence rate of smoking?
   

2. 
   What was the prevalence of smoking at the time of the survey in June, 1995?
   

3. 
   The survey was given to the 110 graduating seniors. How valid are the results of the survey as estimates of smoking incidence and prevalence for the high school age adolescents in this community?