Prevalence rate refers to the number of both old and new cases in a population during a specific time period, which is divided by the total number of cases in the population. Prevalence rates are useful when dealing with investigations relating to chronic diseases, which last for more than 3 months. On the other hand, incidence rate refers to the number of new health related conditions or cases which is divided by the population at risk. Incidence rates are important in studies involving acute diseases, where symptoms of a disease peak and subside within days or weeks and generally lasts less than 3 months.