Frances decides to conduct a second study on concreteness and memory in a laboratory setting. She creates a list of 12 concrete items (e.g., pencil and table) and a list of 12 abstract items (e.g., justice and freedom). Participants view each word for 1 second and then recall them in order. The study shows a null effect, and almost all the participants remembered all the words on both lists. Frances' study has a null effect likely because of -, which led to - variability. To eliminate this problem, Frances should consider making the task