Question

Distinguish between period prevalence and incidence. What is the definition of lifetime prevalence? Explain the meaning of the formula P~=ID

Answer 1

1) Incidence refers to new cases of a disease, while prevalence refers to all existing cases.

The two are related (Prevalence = Incidence x Disease Duration), but aren't the same thing. For example, for an incurable disease like HIV, the incidence rate could be decreasing as prevention measures improve, but the period prevalence can still rise (due perhaps to increased survival rates).

2) Lifetime prevalence is the proportion of a population that, at some point in their life, has experienced a particular health event, risk factor or disease. For example, in a survey, you might be asked if you have ever smoked. Lifetime prevalence is calculated by comparing the number of people found to have experienced the health event with the total number of people studied.

3) If the average duration of disease remains constant, then preventive measures that reduce the incidence of disease would be expected to result in a decreased prevalence.

Similarly, if the incidence remained constant, then developing a cure would reduce the average duration of disease, and this would also reduce the prevalence of disease.