Complex sentence: Most frequently, tests are administered to people of similar ages or grades at particular times in development because it allows research to be conducted on correct to incorrect response ratios for 3rd graders (for example) over time or from region to region.
Split into simpler sentences: Most frequently, tests are administered to people of similar ages or grades at particular times in development. This practice allows researchers to study correct to incorrect response ratios for 3rd graders (for example) over time or from region to region.