Which approach is used when a researcher assigns subjects to groups based on characteristics like age to ensure equality?

The correct approach used by a researcher to assign subjects to groups based on characteristics like age to ensure equality is sample matching. This method involves pairing or grouping participants across different conditions based on specific attributes, such as age, gender, or other relevant demographics. The goal is to create equivalent groups for comparison in the study, allowing the researcher to control for these characteristics and isolate the effect of the independent variable being studied.

Sample matching is especially useful in observational or quasi-experimental designs where random assignment may not be feasible. By matching participants, researchers can reduce variability within groups and enhance the validity of their findings, leading to more accurate conclusions about the relationships between variables. This ensures that potential confounding variables are balanced between the groups, enhancing the integrity of the experimental results.