Comparison of Crude Rates of Two or More Populations in the Same

Comparison of crude rates of two or more populations in the same period or in the same population in different time periods can be misleading because:

A) Individuals are not equally distributed across various age groups in any population.
B) The nature of diseases is unknown and may not be predictable
C) Individuals frequently move to new locations and therefore they are difficult to track
D) The risk of death, or disease is strongly associated with age and is not equally distributed across all age groups in any population