Rates refers to the number of cases occurring during a specific period of time and depends on the population size at that time. Calculating disease rates helps epidemiologists to compare health issues among different populations. The general calculation for determining disease rate is to divide the number of cases or health condition by the number of population at risk during a specific period of time, and multiplying that by 100. However, disease rates can also be differentiated into two different types: prevalence rate and incidence rate. 