Objectives: The objective of this study is to provide examples of situations in which the relative ranking of age-standardized rates depends on the choice of standard population and to emphasize the importance of selecting an appropriate standard population. Methods: Mathematical methods are used for elaborate variance in the ranking of the age-standardized rates. Examples from the international comparisons of suicide rates and regional comparisons of cancer incidence in Korea were used. Results: The worldwide ranking of suicide rates for the Republic of Korea varies depending on the choice of the standard population (OECD standard vs. WHO standard). Similarly, the rank of South Korean cancer incidence rates by region also varied depending on the selected standard population (2000 mid-year population vs. 2020 mid-year population). This phenomenon has been demonstrated using coordinate space and vectors. Conclusions: An appropriate standard population should be selected according to the research purpose since there is no established recommendation. As conclusions may vary depending on the selection of the standard population, it is important to pay attention to the interpretation of the results.