little help, incidence rate with lost to follow-up

hi there,
i hope you can help me. I am studying for a test and there is an answer that i cannot solve...
it asks to calculate the annual incidence rate of a disease, when there are 275 new cases in a year, the number of exposed to the risk factor at the beginning of the year is 2400 and the number of lost to follow up is 960. Can you do that??

please help me!!!


Less is more. Stay pure. Stay poor.
What textbook are you using for the course. Your actions depend on the procedures you have covered. Theoretically, you would need to make assumptions to move forward. The key assumption would be that there was not a systematic error causing the loss to follow-up (e.g., there was no reason that those exposed or unexposed were more likely to be lost), that loss was random.

Given you are comfortable saying the previous, a common approach is to treat these individuals as censored and account for them in the denominator. The actuarial life-table approach would be to calculate the incidennce as the number of events divided by the number of subjects in the cohort at baseline minus one half of the losses.