Final answer:
The number of new diagnoses over time determines the incidence rate and helps in understanding disease spread and risk factors, while both new and existing cases define the prevalence rate, reflecting the overall impact of a disease on a population.
Step-by-step explanation:
The calculated number of people newly diagnosed with a disease over a period of time is used in epidemiological studies to determine the incidence rate of that disease. This measurement helps identify risk factors and the potential for the disease's spread within a population. The prevalence rate, on the other hand, includes both new and existing cases and is used to indicate the overall health burden of a disease within a population at a given time.
For example, if scientists studying a chronic disease observe a higher number of new cases in the southern United States in 2013 compared to 2012, this could indicate an increase in the disease's incidence and suggest a need for public health interventions. To make such data more comprehensible, rates are often expressed as a percentage or by using a factor (e.g., per 100,000 individuals) to convey the significance of the figures more effectively.