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Suppose there are $6$ people in a population. During $2$ weeks $3$ people get the flu. Cases of the flu last $2$ days. Also people will get the flu only once during this period. What is the incidence density of the flu?

Would it be $\frac{3}{84 \text{person days}}$ since each person is observed for $14$ days?

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  • $\begingroup$ Do you know when each people get the flu? $\endgroup$
    – ThiS
    Commented Jan 24, 2013 at 13:25
  • $\begingroup$ @Thierry: Assume that they get the flu on Monday, Tuesday, and Wednesday of the first week. $\endgroup$
    – user20032
    Commented Jan 24, 2013 at 15:13
  • $\begingroup$ Please only use the "Your Answer" field to provide an answer to the question. Do not use it to comment or reply to comments. Since you're new here, it may help you to read our about page & our FAQ. $\endgroup$
    – gung - Reinstate Monica
    Commented Jan 24, 2013 at 15:44
  • $\begingroup$ larry, can you confirm that these two accounts, 20010 and 20032, are yours? A moderator will merge them but you'll still need to register your account once and for all. $\endgroup$
    – chl
    Commented Jan 24, 2013 at 22:56

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The study population is at risk, that is to say that its members should be able to contract the disease in the study. In general, we look at the first occurrence of a disease in a person. In your example, the 3 cases diagnosed during the study are not at risk from the moment they are diagnosed. The total duration of follow-up for this study was 3*14person-days for those who do not have the disease (3 people times 2 weeks) and say that the study begin the first week on Monday. And the 3 peoples get sick on Monday, Tuesday and Wednesday of the week after, we have 7+8+9 person-days for those who have it.

The true incidence is $\frac{3}{(3*14)+(7+8+9)} = \frac{3}{66 person-days}$.

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